I recently purchased a Sym Jet Euro 50 secondhand. It maxed out at around 30mph flat and 35 downhill. I just took the nylon washer out of the variator and cut off the breather pipe on the exhaust, fitting it with a small muffler. It's loud as hell, but i now hit 35mph on flat and 45mph downhill. I was wondering if there is anything else I can do to increase speed and accel without buying expensive parts??
I don't really know about that particular model, but from what I've read it sounds like they could have exhaust, CDI, and variator restrictions. It seems you've covered the exhaust and vari, so perhaps look into the CDI. If it sounds like a rev limiter when you top out, it's probably limited. The fact that you can get to 45mph downhill might indicate it is capable of revving though.
Tuning the carb, roller or slider weights, and clutch springs are other inexpensive ways to increase performance. Beyond that, you can consider port mods. I suggest reading up before getting into that though.