High compression ratio heats the mix and encourages a fast air:fuel mixture burn.
If the burn is too fast, peak cylinder pressure occurs too early and heat builds up further, encouraging an even faster burn .. the end result is detonation .. a severe strain on the engine accompanied by knocking. (This is how heat puts strict limits these air-cooled engines' power outputs)
A base gasket physically raises the cylinder up. The piston can't compress the mixture as much as it would without the base gasket. So a base gasket is one of the things that determines compression ratio.
The change from a stock 40mm cylinder to 40.4 mm bore raised the compression ratio a bit.
If your combustion chamber volume is 8ccs, the rise in CR (compression ratio) went from 7.28:1 to 7:40 to 1 . This is a minor increase in CR and can be disregarded, imo..
But add no cylinder base gasket to the equation and who knows.. you might have raised CR by enough to enter the heat problem zone...