It is difficult to swap the slant 4 B21 in for an upright B20. You need a special bell-housing to begin with and these are very rare. Most people tend to have this bell-housing fabricated by combining parts of a B20 toM41 and a B21 to M45/46 bellhousing. Then there's lots of other issues that still need to be resolved after that.

If you're going to rebuild the B-20 then this is what you should do. Go to a self-serve junkyard and pull a B20 from an early '75 240. These were the best B20s ever made with much stronger pushrods and I think better connecting rods than what your B20 has. You might also want to grab the M41 tranny from that car because that was also the strongest M41 ever made.

Get some new standard 8.5:1 B21 pistons (fairly easy to get) and bore the B20 block out to standard B21 bore size. Use the connecting rods from the B-20 and you'll have a 2.1 litre 9.5:1 B20 based engine with heavy duty pushrods. It may not be any cheaper to do this than buying IPDs B20 rebuild kit but you'll end up with a much stronger motor in the end. To save some money on the pistons you could get some used B21 pistons and put a new set of rings on them. You might also be able to get new B21 pistons for a decent price on e-bay if you're lucky.

Just my $0.02 worth,

Roj