Performance Mods To 1991 Ford EA 3.9L Multipoint Injection Car

[BRADMAN]

Hello all,
i have no idea on motors. i only understand electronics.
So far i have done nothing to my engine.
To make matters worse (maybe) the car in on LPG.
I want to get some more straight line power from the car.
But i do not want to reduce weight...ie the subs in the boot

Car has a 2.5" exhaust system from the cat back
Car has brand new radiator...i think a better than stock on but nothing too special and has just had head gasget done

What mods can i do ranging from cheapest to dearest to get some more power out of the motor...keep in mind what mods i do now i need to work in future if say turbo is added or whatever.

I have read putting on a Xh snorkle and K & N filter into stock air box helps
I have also read about putting on some extractors and high flow cat converter

are both these true
if not please make a list of mods i should do
thanks all
BRADMAN