I'll beat the crowd and be the first one to say the best way to wake up an old 1.5L is to swap it for a TD.
Other than that, it'll respond to pump mods readily. One could conceivably install a TD pump and reference it's LDA boost enriching device to a nitrous fogger. Spray on, extra fuel added, more power. Would be great for passing or hill climbing. Naturally, the best exhaust possible would do nothing but help.
With all that bed space, you could bolt in a nice Pratt & Whitney JT15D jet engine for a little extra boost. It'd run on diesel, and you could bleed air out of the compressor case to turbocharge the rattlebox up front!
Much like this one I spotted the other day:
