Different gears in different motors

OK, so I've discovered that a lot of China bike engines use very similar clutches and that you can mix and match parts from other bikes to make special design clutches if you know what you're doing.

But now I'm wondering about gearboxes. I've been looking at pictures of gearshafts for various engines and many of them look like they could be used in different motors. For instance the Jinlun 244FMI gearbox and the Quadzilla 253FMM seem to be slightly different gear ratios but either complete gearbox looks like it will fit in either engine, so in theory I think you could for instance use a 244 gearbox in a 253 etc.

I've noticed that a lot of CG internals (156?) look like they would fit in 244/253 engines but I have not been able to try this yet. I was just wondering if anybody knew about swapping gearboxes between engines. I realize there may be some gear ratio differences but that's easily dealt with by sprockets, and some clutches are held on with a circlip and some are held on with that annoying Honda style nut but again that's an easy adaptation if the gearshafts are interchangeable between engines.

My own engine at the moment is a mixture of 244 and 253 parts that work really well. The pistons are YX140 pistons in modified 253 cylinders using a 150 AJS head, and the crank and gearbox are all from a 244 engine. I've basically made a 256 engine of around 200cc and it works great but I'm just wondering what other gearboxes can be used, if anybody has tried this.