My understanding is the 1988 has the BN barrel and slightly longer stroke 0.5mm or something. 88 on wards they had slightly bigger bore and shorter stroke. Think if you mix and match parts you can end up with a 135cc or similar.

Being a 88 won't it have the longer stroke crank? With that and oversized 3mb barrel probably gained quite a few cc's

Normally it's 3 white wires coming from the stator to voltage reg.

Probably the cheapest option but I've no idea what the quality of them is like.

If the speedo works I doubt it'll be the cable, sounds like the speedo unit has suffered some damage, probably easiest to replace speedo unit unless you can find another spares one to cannibalize for parts.

I may have one, stripped all the starter motor parts off my engine. Can have a look if you want?

More mechanical advantage from a longer clutch activator arm maybe? Smoother cable routing?

I believe there expensive due to being a oddball size, something .5mm wide I think.

Tyre size? My speedo under reads 3mph of actual speed, due to different size tyre, if you factor in most speedos over read. Could be 6mph+ difference.

Sounds like it's running lean at some point with choke improving things. You need to work out what throttle position the bog occurs and try richening up the part of the carb that controls that throttle position.