I would say that the head is warped.
Pull the decomp and spark plug out.
Remove the head.
If the gasket is stuck to the block you are lucky, scrape it off. If its stuck to the head its about 10x as hard to get off. Then with no gasket put the head on the block, use 20 of the bolts turned in a few threads as a guide and see if the head sits flat or if it wobbles.
Either get a new head or good used one.
Or you could try flattening it yourself.
A bit of glass or clean flat steel. Lay out a sheet of wet/dry say about 320 grit, use either water or wd40 and get rubbing till its smooth and shines up all over the gasket face. How much care you take will show up in the end result.
I have often used the above method for getting whats left of the old gasket off the head. For some reason they can stick to the head like they are fused there. Using a scraper you just end up digging chunks out of the face of the head.
On the fuel thing.
If the tap is leaking and your head is warped, you will get fuel evapourating easily.
Pull the fuel line off with the tank full and check the tap does not leak. I had one that did that, too me a while to work out why the mower was a pig to start. Turns out it was constantly flooded.
Cheers, Bob.
