I know the REPU and courier where the same in the 70's would a rotary be a easy swap in a 81 courier? Any input would be great. I have been out of the rotary world for about a year now and really wann get back in it.

 

the REPU is actually unique under the hood. The rotary doesn't even super easily swap into the mazda B-series of the same year.

That said - the old couriers are pretty simple trucks and I don't see why it would be that difficult to fab up some mounts to put one in. FC 13B style mounts will probably work easiest.