Thanks for the help! I did find a Grebe on EBay, and did an immediate BIN. Though I definitely overpaid ($35), I checked with all my usual sources before and no one had it in stock (or could even get it some other way), so I guess I'm lucky I got it. I also discovered there's another 1/72 kit for the Grebe by Aeroclub (which I presume is even harder to find...) It might be the same kit under a different label, however. Does anybody know?
Anyhow, I read up on the Grebe and it's basically the Sparrowhawk fuselage plus a set of new wings. Different engine too. The tail fin is also slightly different, but very easy to backdate since it's actually larger. Now I should find out which wings are the closest...

On the Windsock Datafiles, the wings of the Nighthawk/Sparrowhawk appear to be thick and square-ish and with a noticeable dihedral, while the tailplanes have a round leading edge. I would say that the wings are similar to those of the Sopwith Pup, especially in dimensions, although those of the latter have a different side edge and less dihedral.
As it already happened a couple of times, as soon as I finish a complex scratchbuild/conversion/kitbashing, a major model company will inevitably release a fantastic injection kit with oodles of details and exceptional fit... Oh yeah, and it will only cost $15...

I'm attaching a top drawing showing the Nighthawk wings, which are identical to the Sparrowhawk's. Any suggestion?
P.S. Your mentioning the powerplant gave me another idea and I grabbed a Toko Sopwith Snipe for a mere $6. It will provide engine (the Snipe was designed around the BR.2, right?), cowling and the machine-guns, although in all the pictures I've seen, Japanese Sparrowhawks appear to be unarmed. But then again, they were basically testbeds for ship launches and by the time the carrier Hosho entered service, they had already been replaced by the Mitsubishi 1MF-1 (incidentally, also British-designed.) It's quite possible that they never received any armament.