personally i reccomend the 50cc RGD kit. great power and in combination with a pipe and carb u can do 45+ easy.
kitting a za50 is tricky. they are not easy to portmatch and honestly they arent that good of an engine. keep in mind this is coming from the number 1 puch fan boy.
the za50 just isnt as sturdy or reliable as the a35 tomos motor. it will break eventually if you kit it. i just had one lock up on my at 50mph on my monza and im lucky to be alive.
especially since you have such a kickass bike, i wouldnt want to see it get hurt. and i know what you are thinking, "whatever if i break the engine i can get another one". no you cant. you have a collectors item dude and you want the engine vin to match the frame vin.
seriously 50cc kits are the way to go on a za50. i had one on my trike and was doing 40 with low gearing. would you prefer a moped that went 45-47 and was reliable or a moped that goes 55+ and breaks all the time?