Sounds like you got the GEN B16a motor package from the 88-91 model. These come with the cable gearbox (different housing to the later generation). It will require heaps of work to get it running in an EG.

You should of gotten the B16a pacakge from either the EG6, CRX (94 and above) or Ek4 (you need to change a few things). And no, dont need aftermarket mount to fit into the EG

Hope you get it sorted out