Vega - Performance
The problem with a 60 year old machine is we all don't know the condition of the internals unless stripped, checked and upgraded where needed.
The J and Lui 50cc machines are notoriously slow to start with, the engine components were not really designed to do any thing other then 25, maybe 30 miles and hour if you get a good one.
With this in mind, the more performance you want, it goes without saying the more you will need to delve deeper into the engine the faster you want to go.
The crankshaft, bearings, oil seals, and clutch and the heart beat of the engine, and if putting on a cylinder kit or other work, these need to be checked, replaced or upgraded if need be.
The first step is minor, and may give you another 1 to 2 mph, fit a Big bore exhaust. You may need to go up one to two sizes on your main jet, and adjust your air mix screw if needed.
Other then this, you should start looking into adding cc, by either using bolt on cylinder kits, or a little more tricky is original 100/125 parts.
The bolt on kits from Casa (80cc) and GT (110cc) are probably the easiest to go for, as they are designed to specifically bolt onto your existing engine and components.
If you convert from 50 to 80/100/125, you will need the corresponding crank, cylinder and piston from the relevant donor machine. You will also need some slight crank case modifications, due to the larger crankshaft conrod.
We will concentrate on the bolt on kits.
Casa 80cc Casa suggest the stages of fitment are as follows. Cylinder kit first X3, then as you want more speed, change the sprocket ratios (X28,X29). The third stage is the big bore exhaust (X42), and finally a carburettor X(14). What you can achieve is dependent on your machine, but with all four stages on a well built engine would would expect 40mph.
GT110 Bolt on kits for the 50cc, 75, or 100 kits are all available. The GT kit is a reed valve engine, and gives good torque and increased standard type power, by that we mean no excessive revs or power bands to deal with.
As per our opening statement, the GT kit would need your engine components to be in great order.
A number of 50cc machines have a two plate clutch assembly, you should consider upgrading to a three plate system. This would involve a new rear sprocket and spider, as well as the additional clutch plates.