tuning a bog standard gunpower stealth!

I have a bog standard gunpower stealth in .22 and it’s a sub 12 ft/lb version with a 12 inch barrel, and it shoots putting it plainly rubbish! but im getting it put on my fac and would like some tuning tips from you guys please, i.e what weight hammer should i use?? what size/strength of spring to use, and i know i would have to change the valve, and get an adjustable top hat, also the valve spring is quite thick in this do i need a thinner one to give more air flow?? And last but not least what sort of power can i expect to get from a 19 inch barrel max??? As then i would adjust the power down to give a good number of shots but still good power. thanks guys.