A simple start would be to upgrade it to RS1800 spec. That means robbing the ECU, inlet manifold, throttle housing, air flow meter and camshafts from a 130PS spec model and having those fitted. That should give 120-125bhp on a healthy option.

Further upgrades are an aftermarket exhaust manifold and system, an aftermarket air filter, then you could go wilder with camshafts, cylinder head work, ECU remapping etc. Then that could lead into some aftermarket throttle bodies from the likes of Jenvey etc.

Another option is to ignore the standard engine, and fit the 2.0 version from a 93-98 Mondeo. It's a pretty easy swap, and jumps up to 130bhp/130lbsft, which will make the car a lot more grunty. You could then tune that in a similar way, with slightly better results.

There's also turbos to consider, but that can get very expensive!