I have a stock 79 tt, with the exception of front sprocket, Tecno estorial side bleed and I went from 12 to 15. Top speed will be minimally affected, after just a carb change, not much. But the punch delivered off idle and really at any change in throttle position. You will notice a extra kick. Derbi's eat hills even stock compared to a Puch.
A stock kit, 15 and a decent pipe will make a huge difference. So worth the $200 range price tag on new parts. Do it and be happy, even as a novice, if you are, these mods are simple.