Hi all just had the ebay itchy fingers and brought a 1976 sherpa 125 rolling chassis, I'm wondering if the frame is compatible with the bigger 250/350 engine's or is built especially for a 125 motor. I know the wheels are 1inch smaller than the 350 etc but that's about all the information I can find on frame dimensions. I have two potential motors to fit in the roller, a yamaha 125 aircooled 4t the type that comes in the tyf or a pit bike engine. I'd like to fit the yamaha engine but there will be a fair bit of fabrication as it's probably a little taller than a 125 air cooled 2t, but if I come across an original bultaco engine of a bigger cc I might go down that road cheers. 

The 125 Bultaco Trials cylinders are centre exhaust port while the larger Trials Bultaco engines are side port which the twin front downtubes on your frame will not suit. Larger motor Trials Bultacos have single front downtube frames

 that rules out sourcing a bigger cc motor then,iv been doing abit of trawling  tonight and did notice that in the photos. I found that the crankcaseing are the same on the 125 as the bigger cc engines, and on a old thread someone has bored a 125 to 185 but for the money that would all cost its going to get a transplant .ohh well it was brought as a project for my spares anyway cheers for the info.