Hello and welcome to the club,
A good start is to run some BG44K through with a tank of fuel. I noticed a near instant improvement after using it. Smoother tick over ect.
You cannot get any more than minimal power gains(if any) on these 6 cyl engines. Only thing you can try cheaply is the usual k&n filter or after market induction kit.
There are literally hundreds exterior/interior mods on the market, just have a browse through flea-bay and the forums Richard.
regards. Ian.