Simple thing is just to use the whole bigger motor as all the engine mounts are in the same place. I believe MK5 motors had the small diameter crank which pretty much limits them to 60mm stroke. Mk7 went to a larger OD crank and the first with the 64mm stroke 350/360 whereas the MK6 bigbore was only 325cc as they had 60mm stroke. There are different stud patterns for the cylinders as well which makes things a bit different. Later on around MK12 the 250 and 370's used the same the 64mm stroke and pretty sure shared the same crankcase.
You could convert your 400 Maico back to 250 though as the 400 and 250 cases are identical but you would would need crank, barrel-piston, head and primary drive chain sprocket as well as clutch hub.
