IMO lapping valves will not do anything for you and would be a waste of effort.
Lapping is intended primarily to provide a perfect valve seal after the valves and seats have been ground or machined. Think of it as a substitute for 'breaking in' a valve job. If a valve does not seat properly after grinding, lapping valves will show the problem and, if minor enough, fix it.
This saves the risk of running an engine with a leaky valve and possibly burning it.
With that much mileage, your valves are either sealing well or are causing problems due to worn valve guides, or are burned or worn. Lapping will do nothing in any case.
IMO it would be very good to remove the head, inspect the ridge or wear on the cylinder wall, and get the head rebuilt. This includes removing valves, grinding seats, valves, removing deposits and new valve guides.