in order to make this 1200 a 50 hp 1600 you'll have to change the crank, pistons, barrels and heads. only think left will be the original case (which isn't a bad idea but you may not get all teh power you need).
if you wish to get more power with little modification you cna convert it to 1300 and dual-port (AR) heads which willtake you from approx 26-30hp to 44hp (significantly more power). you will have to source at least a 28PICT carb for this engine but ideally you'll need a 30PICT. a full rebuild on this engine woudl be a good idea so that you can replace all teh bearings and generally get everythign inside cleaned up and adjusted to that you get the most out of it.
please ensure you upgrade the cooling system to the late model dog house shroud as well so that it doesn't overheat.
i'd suggest you have a talk with asadishaque and he'll guide you where to go and what to do on your car better than any of us online.
leave ALL cosmetics till after most of the major mechanical work is done. concentrate your money and efforts towards bushes, mounts, seals, etc. for the time being.
good luck and keep us posted buddy.