I can’t take credit for this idea.  I saw this on a blog ages ago.  I have since tried to locate the blog and haven’t had any luck so i figured i’d share my experience here on beemersandbits.com.

Hopefully this will help those out there trying to do the same key modification to their /5.  I think /2 BMW’s use the same ‘beetle’ looking key.

Photo lifted from Rocky Point Cycle. They include detailed instructions on how to install this key mechanism

Original beetle looking BMW motorcycle key for a /5.

First you have to remove the old key mechanism. Quickly you start to realize that this upgrade is valid. the contacts and wiring aren’t the best after 38+ years.
Start by taking a long screwdriver and carefully bend the 4 tabs that hold the original key relay in place.

Removal of old switch mechanism

Not much to it. this is the order in which all these pieces went together.

The next step is to prepare everything.  I used some small drill bits and drilled around the old key.  After doing enough drilling, i was able to wiggle it loose and remove it from the ‘beetle’.
I then had to file down the head of the new key to fit inside the ‘beetle’.

Old 'key' removed from beetle. New key shaved down to fit.

Next was finding a way to get MacGyver and keep the key in place while the epoxy hardens.

Key in 'run' position