There is not a great way to do this cheaply. You can increase compression by swapping heads or swapping pistons or rods. To increase RPMs, you'll need to get more fuel and air into the motor and get it out quickly. That can be done with carbs, exhaust, and your cam. What cam are you wanting to run? If you don't have a lot of mods, you won't be reaching the 7500 range easily. The governor will limit you to 3600 rpms. If you remove that, it's recommended that you add a billet rod and flywheel. That will cost around $175 to buy those parts. Then you can remove the governor. Your stock valve springs will then limit you to around 5500 rpms. If you move up to at least an 18 lb spring, you'll see better results, but that will probably get you to around the mid 6000s (if you've rejetted the carb and added a more free-flowing exhaust. If you want to reach 7500, you'll need a bigger carb.
The cam opens and closes your valves. There are many different cams that will accommodate different needs. Some cams will provide a lot of lift (which opens your valves really far) and others provide more duration (opening the valve sooner or keeping it open later). There is definitely a science to this engine stuff. I am not a professional builder, and my total engine builds number only 4 or 5, so you'll definitely want some more feedback on this.
The bottom line is this. If you want to go really fast, you're going to have to spend some money. If you're building a mildly modded engine, you should look at a mild cam. Dyno Cams has a lot to choose from. I got my clone cam for around $50, and I'm very happy with it. I think I'm running 265 lift and 236 duration, but I'd have to look to make sure. I have been to the 7500 range many times, but I haven't been higher. I will eventually upgrade my valves and pushrods, and then the bigger 26lb valve springs will be installed. That may help me get a few more RPMs.
I suggest reading as much as you can about how these work. A simple Google search can provide you with some really great websites. There are many great books that can be purchased as well. I have found that hanging out with knowledgable people is the best way to learn. My buddy raced sprint cars for a while, and hanging out with his dad in the garage was where I started learning this stuff years ago.
As for your RPM range, you need to think about what you'll be doing with your motor. Are you racing? Are you riding on a short track that would require more power in the lower RPM range, or are you wanting to run on wide-open roads where you'll have room for higher RPMs?
