Nissan Datsun 120Y

What year is it and what is your engine right now? I know there's space to dump an SR20DET in it but I don't know how extreme you plan on getting. If you have a B210, '74 to 78 series the your engine will be the A13 or A14. If you have the 310, '79 to '82, then you have a, A12 to A15 engine. Of course I think in Asia you were able to get a L16T. None of those engines made big power and the only ones I would spend any money on would be A14E and A15E which were the fuel injected ones. The only way to get real power out will be to do an engine swap like what I'm doing on my Datsun, a 280z by the way. I'm putting an SR20DET in it because I can build a 400hp reliable engine out of it fairly easily.
 
How about forgetting about modding it and giving it to me? 😀

If you don't want to spend too much cash, a K&N Filter and a can of Seafoam will make it increase HP & fuel economy.
 
Agreed with the K& N filter, but for older cars try to source something called "Marvel's Mystery Oil" and throw it in the gas tank and a bit in the crankcase and take her for a spin.
I'm not sure about the SR20DET fitting, but you can find an SR16 five forward in most parts place if you want to comfortably fit something new.