I've never personally put a B20A5 head on a B21, but many people have successfully done this.
The heads themselves are very similar, so you don't have to do any special modification to get everything to bolt up. If you have a B21 block and a B20A5 head, just get a head gasket (for a B21) and bolt the B20A5 head to the B21 block.
There are several advantages to doing this, the first of course being the increase in compression ratio. The final compression ratio will vary slightly depending on your head gasket thickness, but in the end you'll come out ahead of the stock B21 head.
This head swap also creates some nice little squish zones (a good thing) in your cylinder due to the slightly smaller bore the B20A5 head is machined for.
I don't have any official documentation, but many people also claim that the B20A5 head flows slightly better than the B21. If you're attempting to do a naturally aspirated build up, using the B20A5 head makes sense.
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