I think you would need a different top mount for the engine and a few other pieces but it's fairly straightorward. IMO it would be as well to drop a valver engine in there. If you went for a valver conversion you could re-use your box, engine mounts and driveshafts, but you would still need to get a 16v fuel pump, loom, ecu and 16v suspension and brakes.
EDIT: Also try doing a search on here and on retro-renault.com as I asked this question on both sites ages ago and got quite a lot of info about the conversion.