That looks a pro job...
I did not think of a C15 motor its just my mate has rebuilt several small brit single engines with higher quality bearings and upgrades. He did suggest a Triumph 350 saying it would fit in fine, he has built Tribsa's and the like in the past. The alignment is out of my league so only going on advice really. The Montesa 248 engine is difficult to get some parts for (common bottom end with 349) so looking to use this long term, I would buy a 348 or 349 engine if one was going but you dont see them often.
Thanks for the reply