with 20k,

my suggestion is to go with A31. parts are much cheaper than KE70 parts and you have wide platform to choose from. engine you can go from RB20DET, RB25DET, RB26DETT, 1JZ, 2JZ or SR20DET for weight savings. with the heavy body weight and LSD available in abundance, you can get the car drift worthy and street legal... not to mention heavy body = easier to drift.

120Y, if uses SR20DET, all you need to mod is the engine mounting and also the gearbox mounting and the gear lever... it sits easily 5 inches further to the back due to larger gearbox... on the engine bay, you might need to shift the radiator to the front of he panel to make room for the larger engine block. this means to sacrifice your aircond, or fabricate new mounting for the aircond condenser.

for the rear axle, you'll need LSD, since the original is live axle, you may mod to sub frame from S13 or R32. this means you need to heavily mod the rear mountings and basically cut the rear boot floor board in half. for front suspension, you can use S13 or A31 as it's interchangeable for top mount, but you need to mod a little to accomodate the brakes. since with the added HP, you might as well use S13 brakes and calipers and rear from S13.

with the added HP and weight, you need to reinforce your under carriage or else you'll twist the body to shreds.. this can be done by welding a half inch steel plate to the current under carriage from the firewall to the rear pan hard rod mounting (under rear seat).