Sorry I haven't gotten back to you earlier, I'm not on the board much these days.

Your 87 Si has either a D15 or EW3 coded engine. There should be a blue anodized ID plate under the hood in the engine bay that has the engine serial code on it, otherwise you have to find the markings on the engine block, near the transmission joint.

Unfortunately, even if it's a D15 coded engine, it's not the same as the 88-on D series engines, so there's no well-established aftermarket for those engines. There are things you can do, but swapping isn't very straight forward either.

Probably the "easiest" swap would be from a 1G Integra, with a DOHC D-series non-VTEC engine. It requires some creative axle surgery, or a steering knuckle swap to get power to the wheels though. Any swap newer than that is going to be close to 100% custom, don't count on much, if any, aftermarket mounts to be readily available.