Of course you are gonna get the die hards to say that is a bad idea. And they will say that you should re-build the 500, this time using an RM wrist pin bearing & billet thrust washers. But it's your project to do as you wish & there is no doubt that a Honda is a reliable motor. Just remember that there are modifications that need to be done to the 500 to make it reliable as well. Maybe you can put the 500 engine in the TRX frame?