From my experience with a 66cc engine;
1. Sprocket makes a HUGE difference, it is easily the most important mod you can to with any of these motorized bikes. Looks like you are using a 44T, that’s too big. On my 66cc heavy hybrid bike build, I could only top 28mph with a 44T on a good day. I’m not sure how low you should go because your 33cc is probably not putting out that many HPs and torque, but at least drop down to a 40T (I use 38T). The switch from a 44T to 38T already made an enormous difference for me, top speed went from 28 to 35, RPM and vibrations significantly reduced (better for engine to run at low RPM) and better fuel economy.
2. Exhaust also determines top speed but mostly acceleration and torque. It’s the second most important mod. Stock muffler is nice and quiet but very restrictive with its baffling, I bought a 20$ straight through muffler from EBay (just look up 2 stroke muffler) and with a little exhaust cement and angle grinding it fit fine over the stock. My acceleration went from being dull to having to hold to stop myself from flying off! Also, it really improved low-end torque, meaning I could cruise around at low RPM and throttle, saving gas and making for a nice smooth ride. It did improve top speed a bit (few MPH). Just warning you though, it is louder than stock but much better than expansion chamber, which are stupidly loud and hurt my ears. An expansion chamber could boost a bit more, but the extra noise is a big turn off for me. If you want the best of both worlds, you could get an expansion chamber and attach a muffler
3. Carburetor adjustment - you can get better performance, but is a challenge. If you are willing to spend hours messing around with them, that’s fine, but I’m warning you they are a pain sometimes and can lead you in circles. Porting is simple and effective. I removed the stock air filter on mine and replaced with high flow custom filter I made, and that boosted my top speed by at least 5 mph or so. Restricting the carb’s air intake is easier way to kill performance; it will work fine at low RPM but once you rev up it will just suck. Wide open intake or high flow filter will let your carb reach max performance by allowing it to pump as much air/gas mix into the engine as possible.