Speedy depending on how bad the thread is it might be best to pick up a later pull start top that uses a cup. The cup is thin and may leave enough thread to get the nut to hold. Loctite it on, doesn't matter if you can't get it off later, it is only a Briggs so nothing much to loose
NormK, I believe what the OP is referring to is the threads in the starter clutch being stripped; something I have seen several times and not damage the crankshaft threads as the crankshaft threads are steel and the starter clutch are aluminum. This is usually cause by over torquing or cross threading and from not use the proper tools. Many home mechanics just use hammer and punch to loosen and tighten these. I have even seen a couple crack flywheels as a result.
As for the conversion you mention it depends on the engine setup as to if starter cup setup will work. Some the older engines do have conversions possible and other not as replacement starter cup recoil is not available that would fit. It also requires the cutting of the crankshaft stem on those that can be converted as the stem would be on the way of the new recoil.
