That's what I wanted to do. Find a CRX, and buy a D15Z1 Engine and ECU. Way I figure, I can get a CRX for $500-$1000, Engine for $500-$600, and ECU for $120. Hopefully this will push me into the 50 mpg.
The only problem with a CRX is that they tend to be somewhat expensive and hard to find (for an older car, that is). Also, they tend to be hard to find in good shape. So you might not be able to get one in good shape for $500-$1000. This is particularly the case with the older HF models, which are highly desired by the go-fast crowd for their light weight (which would also help with fuel economy). A 1988 DX might be a good model to look for. Like the early HF models, it is light. But fewer people want it because of the throttle body injection. However, the DPFI-MPFI swap is really not that hard. The difference is even LESS pronounced if you are also upgrading to OBDI (which you will have to do for a D15Z1).