I got 90 prelude si with a b21a1 in it block got a hole in it can I use a b20a5 or 6 with the b21 head and it be a simple swap ?

 

sorry I didn't catch this sooner. yes you can. just make sure to use a b21 head gasket, and you need to swap all external parts over from the b21 block. meaning all brackets and motor mounts, water pipe, etc. Be aware though, using a b21 head on a b20 block lowers the compression a little below what the b20 originally was. so you may notice a mild loss in power.
If you want to get crafty however, and spend a little money, find a set of good b21 pistons and have the b20 block bored to fit the b21 pistons, and then assemble the block with all new parts. you will then have basically a brand new / rebuilt b21 but with steel cylinders that won't destroy the rings fast as hell.