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Both motors should be the same size and interchangeable on the Razor MX500 dirt bike frame, however I am not sure if the sprocket is the same distance from the motor case on both motors because I am seeing a 1-1/4" long shaft on the original MX500 motor and a 1-1/2" long shaft on the 36V 750W motor so that would be something to check. The case length specifications between the two motors are the same on paper but I would also double check that to make sure. The 36V 750W motor has a 5-1/4" long case.

The original MX500 control board is rated for 30 Amps which is enough to power a 750 Watt motor. However I am not sure if you would notice any extra power if using the original controller. I would say that with the original controller you may or may not notice extra power if upgrading the the 750 Watt motor. If you do not notice extra power then you could upgrade to a 1000 Watt 40 Amp controller which would give the 750 Watt motor more power than the original 30 Amp controller does.

If anyone else on this forum has upgraded their MX500 to a 750 Watt motor and knows more about this than I do then please let us know.

A bigger controller and motor alone would not give additional speed unless the gear ratio was also changed.

To gain more speed the combination of a bigger motor and controller plus a smaller rear wheel sprocket would do the trick.

Here are some gear ratio calculations I made to compare top speeds running the stock 11 tooth sprocket and running a 13 tooth sprocket.
Here is the calculation with the 11 tooth motor sprocket:

And here is the calculation with the 13 tooth sprocket:

If you want the bike to have more than a 21 MPH top speed then the rear wheel sprocket will need to be replaced with something smaller, but if you are okay with 21 MPH then installing a 13 tooth sprocket on the motor will do the trick.

Please let me know if you have any questions.

The original Razor MX500 rear wheel sprocket has 80 teeth and its motor has 11 teeth. If you modify the bike to have a 65 tooth wheel sprocket and 13 tooth motor sprocket then a smaller chain will be needed. Whenever new sprockets are installed it is highly recommended to instal a new chain along with them. Because chains stretch and get longer over time, if an old chain is used with new sprockets it will cause the new sprockets to wear out much faster than they should. New sprockets used with with old chain can also be noisy and the chain may want to fall off when the bike is being driven.

For gear ratio calculation purposes the Razor MX500's size 2.50-10 rear tire has a 15" tire height. Here is a gear ratio and MPH top speed calculation I made with a 65 tooth wheel sprocket and a 13 tooth motor sprocket:

Our gear ratio and miles per hour calculator is located on this page: http://www.electricscooterparts.com/motorwheelgearratio.html

I am glad to be able to help. Please let me know if you have any further questions.

I do not know exactly what size #25 chain is needed for the sprocket modifications that you are planning to make. I do know that you will need the heavy duty #25 chain though. We have a chain length calculator which can help determine the chain size needed on the following page: http://www.electricscooterparts.com/chainlengthcalculator.html

It can sometimes be difficult to take an accurate center measurement on the axle though. To play it safe you may want to consider buying a little more chain than you need and breaking it down to the right size after you receive it. That way you can be sure that you do not buy too small of a chain for the project.

Chain can be broken with a chain breaker tool such as the one that we sell on our tools page - or with a lot more work by using a hammer, punch, and bench vise.

Once you do figure out what chain size is needed it would be really cool if you could post the number of links here so other people doing the same modification will know how many links of chain to order.

                       Chain Breaker Tool

I just had another idea. You could use your old chain to determine how many links are needed by wrapping it around the new sprockets until it meets and counting how many links it takes. That would require placing a second order for the chain though so you could not get the bike working until after the second order arrived. The fastest way to do it would be as I suggested above and order a chain that is a little longer than needed and cut it down to the right size.

Please let me know if you have any questions.