As standard, the TT500's do have a lighting coil, but it has a fairly limited output. An XT500 lignting coil can be mounted to the TT stator plate (they are the same part as the XT stator plate) or a complete XT stator plate assy fitted and use the TT flywheel. This will allow the the XT500 wiring loom/system to be a direct fit.
In the period a accessory speedo kit was available for the TT500's from Yamaha. The front hub on the TT500's are exactly the same as those used on the Yamaha IT series enduro bike, and as such, the IT speedo drive unit can be fitted to the RH side of the front wheel, allowing an IT speedo to be used, which is pretty much what the accessory kit was. I know that replica speedo brackets etc are available from KEDO and their agents in the UK.
Alternativly, you could fit a XT500 front wheel, which would give you both a speedo drive and also a slightly better front brake.
Also in regards to front brakes, the late 70's / early 80's YZ twin leading shoe brakes are the same size as the TT500 front brake, and will fit into the TT500 hub. There may be some issues with the brake plate securing lug though as the YZ all used leading axle front forks where the TT500 has straight axle forks.
The TT/XT (and possibly the SR) 500 countershft sprocket covers are still available from Yamaha. I have had several engines where the mounting lugs have been broken from the crankcases and have fixed the problem by welding small aluminium boses onthe crankcases as replacements then drilling and tapping them to 6mm to suit.
There are several seller selling XT/TT parts (mainly KEDO gear) on EBay in the UK. I have used them along with some of the NOS sellers in the US and allways had good service.
14 out of the 66 bikes in the shed are 500 Yamahas! No, I don't have a problem!