I changed mine a long time ago. Mine was a 500R originally, so it was half way there already. The only reason i changes was because a friend was breaking a 126 and I kept having trouble with the dynamo, so it worked out convenient.
The most complicated part of the change is converting the wiring from dynamo to alternator. This is simple, but you need to follow instructions, that are readily available both here and on the 500club UK forum. Other than that, it was just the drive shafts. Starter location didn't affect it nor did size etc. The engine mounting may also pose a small challenge, but again, this is well documented on many websites, so not difficult with correct instructions.