The ideal way would be to measure bore then measure the old piston see what the clearance is for those. If you run road racing clearance could be somewhere around .003 have run well at .004 but with the larger clearance tends to wear by the exhaust ports but is ok if you keep an eye on it. On sprint only engine I will start just over .002 and break in for a long time. Some people like running the hard gold ring and they do last problem being they tend to wear the block. so thats your choice. If you take the piston out and it still has cross hatch and is round just some clean up with some fine sand paper and your good to go. If you have to hone it that should be done by a pro with a torque plate and special stones. While it is apart you will want to clean carbon from ports but dont use anything that would damage the aluminum as this could be a tech violation. As far as the engine that failed this is an ideal time to learn. Take the upper end apart look that over then see if the rod is blue at the big end the rod is junk if it is. if not locked may have a main bearing failure. To rebuild is easy. Until you get some hands on would leave that to the pros You can just send the crank out.Dont be surprised if the pto half is worn. to take apart you will need a rotor puller and allen wrenches after the rotor is off pull the allens and split the cases face the halfs face down in the oven or hot plate and after heated the bearings fall out make sure its clean and and drop in new bearings do not hit them. There should be some endplay on the crank ideal is about .010. There is no timing to adjust just use a biz card to set the magnet on the ignition. Seals should be able to be pushed in by your fingers keep them straight. Where the pro`s have it is when they take a new engine they square everything up but if yours has been done it should be ok. Hope this helps Have fun.