I have never done valve work on a four cycle model engine. Full scale engines, yes. I would use a soft Dremel wire wheel on the valve stems - easy does it. Do not touch the valve seats. I understand that the valve seats in model engines are cut in the aluminum head material. I would purchase 400 and 600 grit loose abrasive dust (lapping compund on a web search). Do not use automobile valve lapping compund because it's too coarse. Mix the lapping compound with water thined corn syrup. and apply the 400 grit to the valve face and gently rotate the valve in it's seat with the eraser end of a new lead pencil wiped off with lacquer thinner. Lift the valve 90 degrees and rotate and again until you have done 360 degrees. Your valve seat will probably have an even grey tone, do this again with the 600 grit. Apply a thin coat of Prussian blueing or cobalt blue artists oil base paint, available at an art supply store, to your valve face and insert the valve onto it's seat and twist the valve. If you remove the valve and the oil based paint has left an even blue ring on the valve seat you are done. If not, lap some more. Do not over do the lapping because you will wind up widening the valve seat which is good for heat transfer but is not as good a seal as a thin seat. Don't overdo the lapping as the valve seat is aluminum, most other engines have a hardened valve seat insert if the head is aluminum.

Bill