Is it possible to swap a 3rd gen engine into a 1st gen rx7?

If to wat do i need.

yes its possible

there is many things you'll have to consider before doing such swap
whats your goal in power.
whats your budget
what do you know about this kind of swap
do you wanna swap the 3rd gen engine in a 79-80 or 81-85 ? and if its in an 84-85, is the car a gsl-se (13b fuel injected)
once you will have answer those Qs i'll be able to help you.

also think about using the 2nd gen turbo engine, it really can get about as much power and it would probably be cheaper to install... btu it all depends what you want out of it

twinturbo 13b engines from cosmo are also used alot in FC swaps instead of using T2 engines.
they have smaller turbo than the 3rd gen rx7 ones, but they have bigger exhaust port and a sexy intake manifoil