How to fix my 1986 briggs&stratton 12 hp synco balance?

I lose spark after it's been running for about 10 minutes . Theres gas,. it turns over. If I let it cool for half hour i will strart again. But the same thing happens again The riding lawn mower is a 1986 montgomery wards 12/38. I've had this mower for 10yrs, and I never had this problem before. Does this model have points?