also is i risky buying a second hand generator.

I would say its risky unless you can buy with some guarantee its OK. The one I bought from Electrex was quite expensive though. So it depends on your budget. You could get it tested and re-wound. Try west country windings.

right guys, iv investigated and the bikes got the wrong rectifier unit fitted, the bikes a j model but its got the l/n models unit fitted. i plugged the new/correct unit in and gets really hot compared to the old one, is this normal

Yes, if its a shunt type (usually marked SH) it will get warm to hot depending how long the bikes running. The unit turns unused current to heat. Mosfet types generally run a lot cooler. My mosfet unit barely gets warm. Have you checked your stator yet?

not had a chance yet to be honest, just tried the easier cheaper fix to be fair. the rectifier on the bike is marked sh but in the manual its the other one marked 30se1. hope im not sounding too thick