They're good stong engines, its a real shame there isn't more aftermarket for them. Their main problem is their breathing ability or lack there of (log intake). So yea, headers and a carb increase will help a lot.
These are educated estimates, your results could vary either way. Your stock motor makes about 85hp at the wheels. Just installing headers (duel 2" single 2.5"), a new carb such as the progressive Holley/Weber 5200 (270cfm), and an ignition upgrade (duraspark or pertronix) will get you about 125hp at the wheels. See what I mean about breathing?
This is where I'm at right now as I like my 25+mpg.
160 net hp at 4800 rpm with a bigger cam, direct mount 500 Holley or Weber 38/38, and a set of headers, Clifford port divider, late model log head port and polished (these heads have a bigger intake valves but milling is needed to bring your cr back up). Basically doubleling your current power, so yea I say they are worth it.
This is just to give you an idea of what is possible:
Go an even bigger cam, get some better in duction in the form of an import Argintina head, OR 2v Aussie head, (these engines are much more popular overseas) or a bolt-on 3 triple SU carb set,--- there could be 210 hp at 5500 rpm with ease. The exhast becomes the limiting factor here, and the cam must be specialised to allow good flow to balnance the intake flow.
By spending a LOT of money, you could get up to 245hp. Aussies do figures like this regularaly with their little 202 Holdens.
Nitrous oxide and Turbo are also great ways to add power but its still fairly new gound with these engines.
You should be able to bore the block same as any engine. Pistons might be a problem but they are the same as the ones in a 2.3 so it might not be so difficult.
http://www.fordsixparts.com/
This guy sells a lot of parts he imports from Australia.
http://www.stoveboltengineco.com/index1.htm
Sells that Holley/Weber that is so popular
You may want to look into installing a T5 from a 4cyl fox body.