I know a fellow with a rotary in a Viva, and the only problem he had really was how short the motor/transmission was... That is by the time he moved the motor back far enought to get the shifter into a reasonable stretch for comfortable shifting, the motor was buried in the firewall, meaning he had to cut a great whopping hole there!
Seems a shame to have to modify a firewall for anything other than a V8 swap, but you'll have to check things out for yourself