Alright, now we know that your'e in ZA, you know little about Datsuns [for now] & you'r short on cash.

You have a Datsun 1200 & you want more grunt than a 1200 delivers.

The A14 is relatively easy to find in ZA as they are still being used in Bakkies, but A15's seem to be quite rare & would be more expensive if you could find one.

Here's the deal.
The original 1200 gearbox will fit ther A14, but is regarded as a bit weak behind the larger motors. None the less, if this car is to be a driver, & money as a real object, then use the stock gearbox but do not abuse it & it will serve just fine.

If your budget will stretch that far, then get the A14 with the factory four speed attached. These gearboxes are a little stronger & will cope with the engine even when used in a 'spirited' way.

If you shop wisely, you might find a Bakkie engine with the five speed. These are really just a four speed with an overdrive & this will pick up your speed at any given RPM thanks to the OD.

An engine with more torque will allow you to run a larger diameter rear tyre & this too will pick up the pace a little to at any given RPM.
There may also be options with the diff ratio if speed is still an issue as there was a 3.7 to 1 diff offered in the B310 models which I presume you got.

If you have bigger plans for the future, then see if you can get a look under a Bakkie rear end as we got the larger differential from the larger models with L series engines in our Utes, so I think that you did too.

These larger rear ends will go under 1200 Passenger models with little work & offer greater strength as well as higher diff ratios if the right models of Datsun passenger cars were sold in ZA to act as differential donors. The original Bakkie differential ratio is lower than passenger model 1200's, so a swap is needed to pick up the speed.

So,... will the larger A14 engine give you more speed? In stock trim, ...probably not, but you will get to those speeds that it can reach more quickly, it will require less pedal to maintain highway cruise, it will hang on when climbing up hills much better, & when driven right, can easily produce better fuel ecnomy as well.

Your options are plentifull so check them out.