The CA18 is VERY similar in how it's wired up to the SR20 and the KA24 that came in there. The wires are different colors, but the concepts are all the same. There's a few wires that need 12v+ and a few grounds. Starter wiring should be pretty comparable as well. That being said, I would talk to https://www.wiringspecialties.com/
. The wiring on that CA is at least 30 years old now and pretty crusty. I'd bet that you're going to have a lot of broken plugs and bad insulation on the wiring. They could also probably make a wiring harness that would let you plug the CA into the D21.