The VIN will tell you if the shell was originally an HF or not. I don't remember the details, but you can find them in one of the FAQs on
The CRX Community Forum :: Index .
All of the D-series motors from 1988+ Civics and CRXes will swap into an 88-91 CRX shell. The 83-87 CRXes are not as easy to swap, because the HF and DX ("standard") models used a carburetor and had a different fuel system from the injected cars as a result. The 1st-generation Integra motor will swap into a fuel -injected 1st-gen CRX pretty easily, but not into the 88+ ones.
The HF models were the lightest, the DX (or standard) the next, and the Si cars were the heaviest. The later cars are heavier than the earlier ones, as well. The 1991 Si weighed in at just over 2100 lbs, while the 1st-gen HF was around 1650 lbs I think. (Less than 1800, I am sure.)
The HF transmission is one of the keys to getting good fuel economy. It is geared very tall indeed, with tall gears and a tall final drive. They are getting harder to find because people with other Civic-based cars like to swap in the tall CRX 5th gear for their shorter ones.
The ZC motor is one of the more powerful early D-series motors. The DOHC ones made 130 HP, as opposed to the 105-108 HP in the most powerful US-market CRX and Civic. It's a drop-in for a 2nd-gen HF or Si, and makes MOAR POWWAH! That's why it's a big deal.
To convert to OBD2, you generally need a motor from a 96+ car, complete with engine management system. The extra hassle of adding the sensors and making them talk to the OBD system is generally not worth it, because you'll be adapting the sensors to the motor, hooking them up to the EMS, and adapting that to the car. Much easier to leave out one of those steps, plus the easiest way to get all of the OBD2 sensors and such is to get them off of a 96+ Civic. Easy enough to get the motor as well.
There are cable-to-hydro kits for the clutch linkage, so you can use the later hydraulic-clutch manual transmissions if you desire. It is more expense and complexity, of course.
Using the VX engine might give you some really stellar numbers, especially if you can find an HF transmission to hook it up to.
-soD