I'm thinking stepping up diff gears, adding a header with an improved exhaust with a turbo muffler and opening up the air intake with more air.
I had a '91 4 door Sidekick with a 5 speed (5.38 diff gears stock) that I ran 225 size M/T tires on and I added a Calmini header (long tube) and a K&N filter to it. The butt-dyno power improvement was noticeable and I did
not notice any loss of MPG. No it wasn't a dragster... but that isn't what these trucks were built for anyway. I did cruise across the midwest and through the mountains of Pennsylvania to Fort Dix in Joisey many times while cruising at 75-80MPH with
no problem. A 5+HP gain on the 8 valve engine (or even 16 valve) is significant. They are what they are, bless their little hearts.
If you are running 5.12 gears now, jumping up to a set of 5.38 gears ($500?), and adding a header ($300) would be much cheaper than buying a new vehicle OR buying then swapping in a larger engine. The down sides that come to mind would be slightly higher cruising RPMs (100-200 RPMs each gear), your speedo registering slightly slower than your actual ground speed (2-3MPH) and probably losing 1-2 MPG on the fuel economy side.