I see looking back you say it's got a GMC pipe fitted so ignore my 'cut it open' idea--thinking it was the original. Have had various smaller motors [last one was a PE175] that had popped the crank seal out on the primary drive side, it ran but was all gaspy & lean but also smoke terribly so was an easy diagnoses but if a motor popped the mag side seal out [no smoke] it wouldn't be so obvious [apart from the running] & if not thinking along those lines then one can spend a hell of a lot of time chasing ones tale, Been there lots over the years but the good thing is that once solved then you never forget the unusual faults.
One that i didn't solve [cos i was thinking crank seals lol] was by memory a kawy tandem twin [think KR1S], mate called me in to come up with ideas [crank seals or disc valve timing--but no] he fixed it, pulled the cylinders off & someone had used KDX or Yamaha pistons with the cut away inlet which was 'ok' except it had the front cylinder piston was in back to front, hence the cut away was opening exhaust port big time at tdc. Things you never forget. Think it's why the first guy sold it & becomes the second guys problem, then the bike shops problem, & people wonder why repairs cost so much!!.
