nice find, what a present!
Maybe you mentioned the year, but looks like an 81 fairing to me?
Same basic engine as the CT70 and Z50 and even the newer crf's. There have been some improvements over the past 40 years. Rod lengths, electrics, heads etc all differ.
http://www.dratv.com/ has a lot of parts and lots of info on the engine. If you have deep pockets talk to powroll.com, they have been building similar engines since the beginning.
The ATC 110 and 125 horizontal engines are based on the CT90, so I don't think it would be a bolt in affair, different cases. The TRX90 is based on the 70 tho. Some ATC125M had e-start, don't remember the 110 having it, both could have dual range transmissions.
I tried to avoid the chinese engines in my CT70 build but eventually settled on a Lifan 110, 4 sp auto, e-start etc. etc. Picked it up at the local chinajunk shop in pieces for $40, put it back together and now I have a Minitrail that will hit 55mph and wheelie for days, been running strong for 4 years. If you have a local dealer see what they have available, they just replace rather than rebuild.
I like sprees, but know a lot more about the 4st bikes, PM if you need anything.
