As a long time CR500 MX and supermoto racer and rider, I'll tell you that with a few mods these engines can be made very potent and reliable for a fraction of what a 450 hand grenade costs to mod. The mods detailed below are good for 20 rear wheel HP over stock:
I always get a stock base line run on a Dyno before I start mods and I have never seen more than 54 HP at the rear wheel, I usually see around 49hp. The motor makes all it's power in a very narrow power band and tops out at just over 6500rpm. To get that motor to run like you want (usable horsepower) you need to raise the exhaust ports 3.5 to 4.0 mm max, and raise the boost port 1 mill, this will raise your rpm to 8500+ spreading out the power over more RPM, if you have a topend pipe custom made and have the PWK carb + 1.5 mm you will hit just over 9000rpm and few more ponies, but that is it, any more RPM than that and your piston wont last long, 15 hours maybe. There are a few problems spinning that kind of RPM with that motor, 1st, the stock squish band on that motor is close to 2 millimeters which is HUGE, you will need to make that smaller. It is very easy to do the work your self, the ports are so big you can use a file to make the exhaust ports larger and finish it off with a dremel, I would use a file to do most of the work, if your not that good with the dremel it's easy to take too much off. There is also a hump on the bottom of the exhaust port, this should NOT be removed it's there to help the exhaust gases flow better. You will then need to raise the boost port 1 mill, thats it for the porting. Now the head, you will need to take off 1.1 mm making the squish .9mm this sound like a lot because it is, your will then need to increase the combustion chamber size, have a motor machinist do this. Have the combustion chamber CC'd to 68CCs this will increase your Torque and HP and stop any detonation at top RPM.
Now the #2 problem of increasing RPM, the crank, it's is going to be spinning a lot faster than stock and you will have a ton more torque, somewhere around 55 ft/lb's. Having the crank balanced, blue printed, and welded to match the pistons weight will prevent the crank from twisting and keep the vibration down, it also is way easier on the main bearings, Falicon Cranks
http://www.faliconcranks.com/ will do this and it will be worth every penny. If money is a problem you can just have the stock crank balanced and welded. These mods offer really good performance and reliability, you will get 100+ race hours out of the motor before needing a top end job.
In the end you are looking around 68-72+ HP at the rear wheel MAX.
In supermoto race trim I had a friend speed gun the above bike with a 16 tooth front and a 38 rear sprocket running a 17 in tire at 228 kilometers/hour. You will have to run 104 octane or VP MRX01, if your not racing you can run half pump gas and half 104 octane. I know those HP numbers are not what most people expect out of a CR500 but that is the real truth, 65HP stock claimed by Honda is at the crank under perfect conditions and all factory spec are met, realistically 50 HP @the rear wheel stock.
Do youself a favour and install a decompression valve in the head for EZ starting.
