Youre gonna have to post a pic of what you have set up on the engine, or we really cant tell you what your next step would be. If you removed the governor COMPLETELY, (that means the stuff inside the engine, not just disconnecting it), you should notice a big difference in performance. If you didnt, then you did something wrong, missed a step, or are just expecting more than what you can actually get. Once you make the performance upgrades to the engine, you can drop to a better drive ratio like 5.2:1 or 5.4:1 to get more top end, since the new found power can take care of the bottom end. Im gonna assume that your tires say 4.10/3.50-6 on the sides. Whith that size tire, you can run the 6:1 that you currently have, and all the way up to 5.2:1. I ran 13" tall tires on my kart, with a modded gx160, and was able to hit 50mph easy, running upwards of 6400 rpm..... That engine is currently on my mini bike and that thing hits 55+ in second gear, and has plenty of punch in first gear to pop a wheelie and ride it down the block. (69-70 sears roper with 2 speed tranny)
Drilling holes in the factory airbox did nothing for you. The restriction in air flow comes at the intake tube of the box. Its flow capacity is NO GOOD for performance. The intake tract actually slows down the flow, with its sharp, blunt turns. The best set up is a straight shot into the carb, with no restriction in flow at all.....
I done forgot what else I was gonna tell ya.....
