Leave it well alone, just drive it!
I had a 2.5 NA in my Rangie, then a 2.8 NA both slow but 30 mpg.
Its just not worth the hassle , if you want faster you will loose economy, stick a 200TDi in it, bit faster but the Diahatsus are bullet proof and simple to fix.
Ask Graham Dewhurst he is a Diahatsu breaker up north, he is very helpfull and knows his stuff.