You said you have a 91 Si right... NOT the 2.0Si ...
If so, you car is already OBD1; DITCH the OBD0 PR3, and pickup a P28 or any other CHIPABLE OBD1 ECU. Use an OBD1 Distributor, your going to need to change the plugs on the Prelude wiring harness to the OBD1 Plugs that the Newer OBD1 Distributor uses. To change the plugs... match the wire colors, ... thats all there is to it... Also, get a Dist cap for the new OBD1 Dist that is for use with an External Ignition Coil because that is what the preldue uses.
If you think your going to have a hard time with the MINOR wiring task of changing the Plug ends, to match the new Dist, then your are going to have a HELL of a time wiring up the Internal Ign Coil on the new Disrtibutor. Get a Cap for an External Coil, and save your self the extra headache ... Plug and play.
Any more info you need?