I think considering your budget, the "fickleness" of the pinto to cams, the already large size of the ports, the milage of the motor and the cost effectiveness and limitations of modifiing the motor i would agree with the posters who recommend leaving the head on and the block alone.

on your budget 2 twin-choke 45mm side draught webbers would give a good improvement, a bigger single doundraught webber would look very similar to whats on now, if the carb is blended/ matched/ gas flowed to the inlet manifold (and the manifold to the head) it can make a big difference, the air filter is poorly designed and restrictive an encapsulated cone filter/ CAIS helps (PX venom etc) or an open carb with a cool air feed

you will need to re-jet set up the carb/s and it helps if you have someone who knows what they are doing for this

you could fit colder plugs (stock are a bit hot IMO) this would allow you to run a little leaner for better MPG (modded pintos can be gutsy brutes)

the above and a better exhaust would be sensible mods probably within your budget

even if you had money to burn, i feel it would be better spent on a transplant (a 3.0 or 2.8 v6 springs to mind) than spending ££££££££s on the pinto

the volvo/ focus ST 5 cylinder turbo kicks out about 225bhp stock and can easily (and realatively cheaply) be made to put out 300+ - that would be an interesting project, the 2.0L t cosworth engine would be another very sensible option rather than spending silly money/ time/ hassle rebulding the pinto block/ head for realatively small returns