A Porsche 911 engine drops straight into a beetle.
A Subaru lump into a T25 van.
All very true but certainly never as easy as it sounds.
Bike engines go into 126, even Big block Chevy if you are so inclined but always a huge job and lots of head scratching.
As Pete says TRS, or retro500. Tons on this site about swaps too.
good luck.
PS: if you are a recently new 126 owner 'enjoy it as it is for a while you will be suprised'.