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I have seen the threads where the dealer asks a lot of money for replacement keys. If that was a simple transaction, that would be best. Now, I have read the dealers do not carry early R53 keys. I will document my progress with key cutting, programming the immobilizer trip, and the remote.
First, selecting a key to buy. There are many on ebay, aliexpress, and others that all look the same, but they are not exactly.
The original key: Original key blade, circuit board, shell halves including rfid chip, and roll pin. The buttons have broken and fallen out and I've covered the outside of the shell with adhesive-backed vinyl.
The original RFID chip (the 5-sided gray shape inside the left key half) is labeled PCF7930AS. I believe the term "ID73" is used to identify this specific one, but this may be incorrect. Some people accidentally throw this out, this is needed to start your car. The silver rectangular component above the battery on the PCB has "KYOCERA 315K" written on it. I believe this is the crystal designating the remote's frequency, in the US supposedly we universally use 315MHz. I believe UK uses 433MHz
The more common style of aftermarket shell (2 versions): Shells ordered from 2 different sellers on AliExpress. Each was about $4. On the left, close to the original. On the right, allows the indicator LED to shine through which is a useful feature. Both attach the key blade differently, the roll pin attaches the key to a small plastic insert that clips into the end of the key handle. It's a less stable design. On one of them, the key already "wobbles" and I've only just put it together. Also note the base of the key blade has a 1/2 millimeter cut in 5 mm of it, so the end is thinner. The blade might not swap into other shells as easily. Mini badge for the one on the left must be bought separately, and are sold on size: will be listed as "mini badge 22mm".
The true identical copy of the original shell (the one you want): Sold by JingYuQin YanKey. I believe this is direct from the manufacturer as the front of the PCB says "JYQ" on it. This has to be made from the same exact mold as the original, although it also doesn't come with a mini badge. The key blade attaches the exact same way. This version is only sold together with the PCB and RFID immobilizer chip (a PCF 7931). All together was $26. I could not find it available as just an empty shell. EDIT: Search "JingYuChin Mini Key Shell" - you'll find just the shell for around $4.
Second, how do you get into your key and disassemble to swap parts?
Insert a flathead screwdriver into the recess under the keyring hole, and pry off the back cover.
Gently bend outwards one of the two clips on the sides that hold the remote PCB in, and lever out the PCB
Undo the clip holding the immobilizer chip in, and pry it out with a small screwdriver
Remove the roll pin by destructively drilling out the plastic on the front shell and pushing it through, or by grabbing the inside of the roll pin with precisely sized screwdriver tip or other small pointy tool, spinning it as you pull it out far enough to be removed with pliers.
If your original key's buttons are broken off or not working, the button components themselves can be desoldered, removed, and replaced with a 3x6x2.5 mm push button microswitch. I bought a reel of 50 on AliExpress for $2.
Third, cutting
I used a local locksmith advertising they do automotive keys. It was helpful that I had the blanks on hand, and had the metal part of the key removed from the plastic (roll pins removed).
Don't show up in person, call first. The locksmith may just as easily come to you.
It cost $65 per key blank. I had several done. I believe this is called the ilco HU92 style key blank, just like BMWs of the same era (though those seem to use different immobilizer chips)
The process looks like a CNC mill, much more sophisticated than a hardware store cutter for house keys
I saw a service on ebay offering cutting a new blank from a picture of your key, charging $55, and sending you the new shell with cut key blade. They are in Taiwan. No clue which shell they use, probably the cheaper ones.
Fourth, programming
I have yet to sort this out. Both the immobilizer and the remote need to be programmed in different processes.
I have bought a Foxwell NT530 Plus BMW scanner for $160 and hope it will program at least the immobilizers.
Some have reported success with the older and cheaper Foxwell NT510 Plus.
I will report back here with results.
What to do with your trashed old key shell?
I covered the face with the broken buttons with adhesive-backed vinyl "grip tape". I bought a sheet on ebay for another project and cut it to size.
I removed the PCB to swap into one of the above replacement shells
I filled in the area where the PCB was with 1/8 inch of epoxy, so the button holes wouldn't feel like holes through the adhesive vinyl. (Below, far right)
This created a remote-less key, which Mini themselves used to sell, unfortunately even theirs was same size as the key with remote.
I've thought about designing a smaller lower profile fob to attach the keyblade to, that is just big enough to hold the RFID chip, then it'll be a compact emergency key.
For a remote-less key, I could also use a small 2000-2006 BMW X5 transponder key, as below. Looks like the appropriate (ID73?) transponder may fit, but the blade can't be swapped and it has an extra longitudinal cut in it.
