Aldo, I think your design is very marginal if it won't prime itself when the filter element is wet. If you really want to use the filter, you might try just blowing into the top of the tank as a priming system (you must be able to access the filler cap or you couldn't refill it.) Since a careful operator wouldn't run out of fuel unless something unusual happened to cause it, you shouldn't have to blow in it very often, and when you do, you will walk around with a black line around your mouth for the rest of the day, to help you to remember not to run out of fuel next time. Just be glad it isn't a diesel, or you'd have to bleed the system.
As you can see, I'm not very sympathetic. In-line plastic fuel filters on gravity feed systems, located near the cylinder head, are both a fire and a vapour lock hazard. I wouldn't use one.
