Been out in my workshop today and did abit of work on the Montesa.Took the original engine out (no numbers apart from 86 86 on the rear engine mount)I fitted the spare engine 51M15210 the engine slotted in the original mounts so that could be a route I take.The frame number is 21M13371.In amongst the spares was a left hand engine crank case with the numbers 51H00047,looks as though the crank case covers to match this but no internals,other spares included were 3 sets of forks,Montesa 348 alloy tank and seat unit,smaller alloy tank,afew silencers,pair of wheel,swingarm,head identical to the one on 51M15210 and various other small bits and bobs.Now the problem I have with fitting the spare engine is I need to find the matching carb and exhaust so im thinking of selling the spare engine and rebuilding the original but that depends on what damage i find when i strip the engine.Hopefully heres some more pics:
http://www.facebook.com/album.php?aid=252562&id=772753777&l=55f08fac5a