It is the valve guides that need replacing, not the seats. The cost of those jobs depends on who does them, but sometimes the cheapest operator does not do the best work. If you go to a Honda dealer I think that work could cost quite a bit more than a second hand engine in decent condition (either Honda or Briggs). I think most people would hone the bore to 0.010" above standard size, buy an oversize replacement piston and rings from Thailand in a package deal with gaskets, big end shells, and a few other bits, and put it back together. The valve guides may be OK, you can check by measuring the movement of the valve head when you wobble it in the guide. If you don't have a dial indicator to use for that, you can do it by clamping a block of metal to the cylinder's top deck then measuring the distance to the nearest point on the valve head with feeler gauges, at both ends of its lateral movement in the guide. Do that in the direction of greatest movement, of course. If the exhaust guide is excessively worn, you can replace it yourself provided you can find a suitable reamer at a good price. You could try ebay for that.
