The problem with a lot of cars is that they're only as tuneable as they are popular. Because the 1.7 was only used in the Puma, it wasn't that popular, so it never got the attention that some other engines did. The regular, iron-block Zetec was fitted into everything, so got loads of attention from tuners, so there was much more stuff about for it.

I think it depends what you want. If you want to find a few more horses but keep the character of the engine, I would probably try to find some FRP cams, fit a different exhaust manifold and intake, put an FRP map on it and enjoy it as it is. If you want really big power, I would consider an engine swap. The 2.0-litre Duratec from the Fiesta ST MK6 fits into the MK4/5 Fiesta. It's all-alloy so still light, but you'd have 145bhp out of the box. With cams and exhaust manifold, you'd be around 185bhp. With ITBs, you'd be just over 200bhp. The 2.0-litre would give you more torque than the 1.7, and there are loads of tuning parts out there because it was used in loads of cars.