Small Engines (Lawn Mowers, etc.): 1990 Toro 21 side discharge model 16576 mower, toro lawn mower, head gasket

Question
My 1990 Toro lawn mower has gradually been running erratic (miss firing) more and more over the last year.  I changed the spark plug at the start of this season and it worked well until recently when it has actually stalled out completely after heating up.  I suspected it was the points/condenser but to my surprise the Tecumseh engine has a solid state device which I suspect has replaced points/condenser.  Is that correct and if so, after 16 years could it be my problem and require replacement?
Thanks  Greg

Answer
yea that could be the problem but I will put my money maybe on the valves you might have to reseat the valves and then check your clearance valves will make you run like that and sometimes take awhile for it to go completely and then it is usually real hard to start if you can get it started  let me no what you come up with wouldn't hurt to check the valves head gasket if only about 6.00 depends