I would seriously consider swapping in a B21 or B23. In the long run, the OHC engines are more durable and more reliable. They don't have the performance potential of the OHV red blocks, but they will literally run for 300,000 miles with nothing more than oil changes and an occasional valve adjustment. I never had a B20 that didn't eat a cam every 75K-100K. The B230F in my '86 240 is still humming along at 310,000 miles with good compression and very minor oil use (about 1/2 quart every 2,000 mles). It's never been opened up! It also pulls stronger than a stock B20F and runs smoother with its more "modern" fuel injection setup.

The conversion is a little tricky, but it is not uncharted waters. A good B21 or B23/B230 and M46 combo can be had for $500 or less, which is probably quite a bit less money than the cost of a proper rebuild for your B20.