[Unkle Krusty]

CanAm motors have been used before. They are made by Rotax. Both companies owned by Bombardier.
They were 2 strokes, and originally had carburetors. The 250cc single made around 35 hp back in the seventies.

UK

 

[Unkle Krusty]

Whichever engine you get, especially a 2 stroke. Check the piston fit in the bore. What the book says, is not always what keeps the piston going up and down. Different engines and different parts, expand and contract at different rates. That is partly what throws the clearance guides out the door. Too, not all standard pistons are the same size.

UK

 

[Unkle Krusty]

The OP mentioned weight is an issue.
All the older engines weigh more, for the power they are making.
All the old four stroke motors weigh more than the two stroke motors, especially when considering the power.
The OP has found newer engines that may work. I have mentioned engines that have been proven to work, as they have been used. However with one caveat. John Denver augured in. But he probably ran out of fuel.

The criteria from the OP was weight and power, not whether it could be lifted. I think reliability also needs to be considered, and it would be helpful if the glide ratio was good.

We used rubber bands before we could afford engines.

UK