The casting numbers will give you more info on what you're working with to get a better start. For the block, it will be behind the driver's side head, and for the heads it will be under the valve covers. That will give a better idea of performance potential, and a better understanding of what good cam choices, ect are. The more info you give, the more we can help.
NOS will kill any engine quick if not used right...... That is always at your own risk. For that I think it would be a lot safer on a fresh rebuild with the rings gapped for NOS, and a timing controller.
After you get the head numbers, if it's the 76cc castings, my choice would be good 64 cc heads, and a stiffer cam (maybe a 282 if the comp is high enough). It should work decent with that intake, but the carb will be a downfall for you.
All of these choices are dependent of budget, intended use, and goals for the car..... the faster you want to go, the more it costs.