I only bought new gaskets for my B&S quantum engine.
Was about 3 or 4 quid for a pack.
This is why these are hard to start.
Try pressing on the air filter somewhere around the bottom portion whilst priming.
If priming loads still won't work, or it's cutting out then an inspection is nessecairy
Remove pleated filter and kneel/lie down and look into the carb,
when you press the primer bulb, you should see a little squirt out of the jet.
Try again whilst applying pressure to filter housing...
If not the gasket is gone, I tried making some from various materials,which didn't work because I was a bit ham fisted with filter housing removal.
Be really careful removing filter housing as the gasket stuck onto carb/filter housing is made of cardboard like material,
Make another one like it out of cardboard and try stacking onto the old one, and go buy some gaskets from the bay.
Buy some seats for the carb while your at it, and your mower will start with the slightest pull.
The rubber seats are for if the machine cuts out at a later date.
You still might need apply pressure whilst priming the machine the first time,
due to the plastic housing taking a warp or whatever, but that's just a quantum engine for ya.
Probably sortable but not an issue really.
All the best
Tom