As said, 170ish isn't too hard to achieve. Loads can be done to this engine. The thing is, you'll never get massive gains from tuning a normally aspirated engine without spending loads of money. It's not like a turbocharged lump where you can just wind the boost and fuel up a bit, you have to put a bit more thought in to it, and the gains aren't always as simple. That doesn't just count for the I4 either, it counts for all aspirated engines.

If your manifold has been modified well, that's a good start. An ECU chip is generally considered worthwhile, then you can move on to other things. Headwork, modifying the inlet to remove restrictions, cam changes and things can all move things along further. Then you can fit throttle bodies for even more, or go down the 2.3 (or even 2.2) route.

It's a myth that these engines can't be tuned. Parts aren't necessarily so widely available, but they're just as tunable as a Zetec; you just start with more power and torque to begin with.

I think Mountune proved they had the minerals. Race versions of the Escort Maxi peaked at just over 290bhp even back in the late 90s, which is pretty good going for an old 2.0 engine designed in the late 80s.

As said above, the RS2000 forum is a good place to start, as lots of people have tried lots of different things, so you're bound to get some inspiration there