you should be able to id the engine if you go onto vintage suzuki web site and crooks suzuki in cumbria will do engine spares for it. wouldnt mess with ignition will devalue it if you dicide to sell it. If you want to sell it ebay under twinshock suzuki engine or put ad on vitage section of mxtrax
What engine is it and whats wrong with it i have been racing twinshock and evo motocross for last few years so may know where to get spares. Can still get parts for almost all 80s motocross bikes if you know where to look. There is a dutch company who do nos spares for all the jap bikes and can supply most parts
have a look at grastrack.net and grasstrack gb. check ebay for 250 engine 2 stroke the cheapest but could fit 250f best way buy complet bike then sell off whats not needed. belive lee street raced antig yz
have a look at grasstrack.net and grasstrack gb both will have regs when the clubs make them available. As for an engine 250 2 or 4 stroke motocross engine is fine although all the top riders use jawa lay down. cheapest way would be a 2 stroke from a 90s motocross bike should be able to pick one up for around £600 ebay gum tre ect. But if you are doing this make it quick as mx clubs are starting to run supper evo class next year and i can see the price of the bikes going up ie evo is pre 1990 supper evo pre 1999 and the air cooled engine are from twinshock bikes evo and twinshock most popular so higher costs