You don't mention which country you are in so it's not clear just which version of the '120Y' you have. The B210 [1974 to '77] or the B310 [1978 to about '82]?

Engines that will bolt straight into your car include.......................
A12 [1974 onward] 1171cc
A12A, 1237cc
A13 [long stroke '74 B210 version] 1289cc
A13 [short stroke B310 version] 1270cc
A14 1397cc
A15 1488cc

Basicly, any Datsun A series engine that has the distributor mounted near No. 3 cylinder.
A series engines up to 1973 [1000's & 1200's] had a forward mounted distributors & different engine mount positions, & while they can be made to fit easily enough, they will not "bolt straight in"