Are you looking for a bigger engine ? Is that why you want to swap? You can easily upgrade to an a15 a16 or 200b (BIG) depending on what model datsun you have got. It sounds like you must have either a 120y or sunny frame if you want to upgrade from a a12. Sunnys and 120ys and 120ys can hol a15s , they have the exact same engine.a15 makes them very powerfull.The 120ys and sunnys are a light frame so putting abig a16 in it would not be wise and it probly wouldnt fit.