If you've ever looked at both the B20 (which is also called a "pushrod" engine) and the B21-23-230 (which is the red-block or OHC), you'll see that they sit very differently in the engine bay. The former sits upright, and the latter sits canted over to the right (passenger, in the US) side -- I suspect because of the overhead cam, it's taller and thus required the canted position in order to fit under the hood.
The biggest problem, therefore, is how the older engine is going to be bolted to a newer transmission. Smaller issues include supporting engine mounts.
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