Nothing wrong with a b18. Just measure the bores to determine if you need to overbore and then find the appropriate pistons!

Trying to hog a b18 out to a b20 means you have to have it overbored three times at the shop for three times the labor cost and, as mentioned, the possibility of a core-shift in the casting making the whole mess junk (unless you want to pop for a re-lining!).

Then you have the head issue.

A freshening of the b18 with better cam, attention to detail and (my fave) total seal rings will give very good performance.

Don't forget to have the carbs freshened or rebuilt too (needle and jets at a minimum, new shaft bushes possible).

--
Mike!