i was wondering what motors from what years of nissan vehicles could easily be put into a 510 without switching to fuel injection ?

thats a hard question. Probably switch the L-series motors.

since the 510 is over 30 years old you do not need to meat emissions, so basically any engine would work, but you would have to install carburation, and find a simple wat to deal with the electronic ignition, but I think the likely candidates would be a carb'd KA24, L-series. Z-series, and maybe if you've got the fabrication skill small block V8s. The SR20 would not (I think) be a good choice because of the VVT system.

the vvtc is only in s14 and s15 sr20 motors, the s13 redtop and blacktop is a high ported head with no vvtc