1967 MOTO MOWER

NEED A LITTLE HELP ..I PURCHASED A NICE SHAPE 1967 ORIGINAL MOTO MOWER ,,RUNS GREAT TILL I START TO CUT THE GRASS ,AFTER ABOUT 10 MIN,IT STARTS TO DIE DOWN AND RUN LIKE S#%$ T .ANY SUGGESTION WOULD BE GREAT ,,,IT HAS A 10 HP KOHLER ENGINE ,,DON,T KNOW IF THE SPARK PLUG IS THE RIGHT HEAT RANGE BUT IT SURE RUNS BAD AND HAS NO POWER ,,ALMOST SOUNDS LIKE SPARK KNOCK AND IS RUNNING TOO HOT ,,CHANGED THE OIL, NEW FUEL,RUN SOME LEAD ADDITIVE IN THE FUEL FOR THE VALVE SEATS ,ETC ETC ,,IF THERE IS ANYTHING I,M MISSING PLEASE HELP ,,THANK,S BILL I ROTE A HOLE LETTER AND CAN,T SEND IT OUT OF MY POST ,NOT SURE IF THIS ONE IS GOING TO MAKE IT EITHER ,,,,

it could be lots of things.I would check the easy stuff first,clogged fan shroud/screen would be number one after that it could a bad coil warming up,head gasket,points/condensor weak or the engine could be just worn down.I would want to find out if the engine is getting hot
I mean as in overheating (really hot) or just warming up and losing power,hope the info helps KW