as the bottom of an 18r is essentially the same (exceptions being the actual internals, but the block is bolt hole equivalent), there shouldn't be too much involved.
i would be happy to help you swap the physical engine over if you like depending on when you want to do it and where you are.
the only thing i can think of off the top of my head would be the accelerator cable would be different.
you might need to extend a few wires (oil pressure sensor, etc.), vaccuum lines and you could suss out a top radiator hose fairly easily at sprint auto i would say. but other than that...easier than changing the head over i would say (mainly cos it would also involve all the internals in the bottom end as well.)
could be done in a day assuming you had everything ready to swap over, and the 18rg was running fine.
also i am assuming this is an 18rg(u) and not an efi one 18rgeu
do you have another car to drive round in while you do this (knowing that there is always something you need but can't get)???