is it a 91 civic to a 91 crx? or 87 civic to 91 crx.

91 to 91, it's a straight drop, everything is the same.

Mount points on the 87 and earlier civics are different than the 88-2000 civics.
Not sure what year engine you have, but it's like this, 1st gen integra engines, d16a9 I believe, are just like ZC's but will not fit the 88-91 civics/crx. Not sure if there's a family of DOHC ZC that also had different mounts points, I think they are.

If you are using the old Distributor system from the 3rd gen civic, it won't work on the CRX.

Maybe someone understands better now and can give you more information about it.