[Grubby]

There is a tool that screws into the spark plug hole and allows you to use compressed air to hold the valves shut. It makes me pucker, but I have used it with success on several engines. The air pressure actually pushes the piston to the bottom of the stroke - pucker.

The other option is to put the piston at TDC and feed some small cord thru the spark plug hole to hold the valve up. I have not personally used this approach.

There is no way I would pull the heads just for seals.

John