You can put a RWD waterpump on, but to make it work properly you need to change a whole bunch of parts also, which I think is worth it in the long run.
whole waterpump (both halves)
RWD 4A-C or 4A-GE thermostate housing
waterpump pulley
alternator brackets
4A-C or 4A-GE RWD heater tube
The FWD flywheel and clutch is the same and can be used. You just need to put a little bearing in the crank.