I have an Hayterette with a 3.5hp Briggs and Stratton engine (code 94908, manufactured in 1980). As an exercise in learning more about small engines I decided to give the engine a bit of an overhaul, and have come across the following problem: When checking the valve tappet clearance, I found it to be greater than the specified figures - intake 0.012" and exhaust 0.020". I haven't ever lapped valves before but I assume that this would be too much clearance to lap away. The manual for these engines says to cut the valve seat to reduce the clearance, but this seems a little wasteful and I don't have the necessary equipment to do so.
What are the possible causes of this excessive clearance, and what avenues of investigation are there for me to try so that I can remedy it?