The options are endless.

Easiest and cheapest swap probably is a 2,0 Zetec ITB (preferably a ST170 basis), good for up to 200 bhp without touching the bottom end; turbocharging will get you far beyond 300 bhp when you're willing to invest some money on the bottom end.
Mostly the same work but the better engine: 2.0 Duratec HE with ITB, also about 200+ bhp with the right setup
2,3 16V might be an option (RS2000 16V based engine). Nearly nobody tried to get some power out of these engines, but I think with cams and ITBs you'd also see ~200 bhp. Plus it was longtitudinal mounted (in the last Scorpio) from factory which makes it easier to put in when buying a whole donor car.
Cosworth 24V: Over here you get these quite cheap, mostly bulletproof engines, good torque but there's mostly no tuning parts available, especially a set of sports cams would be great to have as the setup of the engine is for a comfortable cruiser mostly.

And there's many, many more options. How about an inline six M3 engine? 286-360 bhp standard.
Or maybe one of the crazy japanese engines (2JZ etc.) I don't know much about?