Would it be a bad idea to put the D15B6 engine that came out of the 89 hf into an 89 crx si? I am wanting to keep great mpg, but not sure if it would work.

I wouldn't recommend it. The CRX Si is about a hundred lbs heavier due to the sun roof and bumpers, so the already slow D15D6 engine would be even more of a dog.

If you're going to do an engine swap into a CRX Si, consider the D15Z1 which comes out of the VX. Or just swap the VX transmission.