Really nice looking bike. Myron's also has details on how to turn your 1.5hp into a 2hp and keep it totally stock. You might get 32 tops. One of the mods is removing the cross pipes in the holes on header part of the pipe you show in your pictures and it looks like that may have already been done as well as the 16t on the front. You can run without a head gasket if you have a good seal. Other than that, a clean chain, tuned clutch, and re-greased wheel bearings are about your limit. It's a big change for a mile or two per hour, but you could go 16x40 gearing. Your takeoff would suffer, but you might get to 33.