Hybrid engine - has this been done before?

[Leon]

Did you have a look at motorgeek.com? There are many individuals who
have done conversions, such as dropping 20vt into the coupe or the
4000.
To your questions: 1 - again, motorgeek, its a cheap mans version of
the 20vt (3B), but usually will include a turbo, not sure if you would
like to go the forced induction way or stay NA. The blocks are almost
identical, the 7A head to a NF block etc. You have to do some math
about calculating the compression ratios though...
2 - they block will stay the same, which means there shouldn't be an
issue of mating it back up to the original transmission.
3 - The transmission of the 4000 is fun, as it is fairly as it is a
close gear ratio. Other options would be I believe the 01E, a 6 speed
but will cost $$. The transmission in your car can handle lots of
abuse, and could break at hard launches once your at the range of
350-400bhp.
4 - Don't know... I don't even how well either car is balanced...