There is an important distinction here that needs to be made very clear.
What you want for road (and motorway use) is torque, not power. When we talk about the eventual (ultimate) "power" achievable with the 1500 engine it is the brake horse power figure we talk about - 150bhp is a very respectable figure for a normally aspirated 8v engine. The limiting factor is gas flow - exactly what you are discussing in this thread. At high rpm and load the 8v head just can't shift the gas in the required volume due to restrictions of the ports and valve throats without assistance from a supercharger.
What you can achieve is more torque at lower revs, the sort of torque that will make an X1/9 accelerate much more quickly but does not alter what happens so much at high rpm. Top speed is not significantly improved and the rev limit is not increased.
A good piece of head work on a SOHC head coupled with enlarged inlet valves and suitable fuelling and exhaust modifications can provide a healthy 130bhp without too much trouble and more importantly much, much more torque than standard.
You can increase the rev limit and get a higher top speed but this is where wear and tear can become significant, if you want to avoid race spec maintenance intervals then I would suggest avoiding more revs (more than about 9500rpm is excessive).
I appreciate this is all a bit contradictory but worthy of understanding. The key to it all is the equation for converting torque to horsepower. The equation is (torque * rpm)/5252 - so when the revs are 5252 the torque and power are the same figure. Below that rev point the torque curve will be above the power curve and above vice-versa.
For a road car with an 8v head you can expect the torque to tail off significantly at the top end of the rpm range. Reducing the swept capacity of the engine will help, increasing it will make the situation worse. Changing the cam profile can help - allowing for more lift and duration but for road use you are limited on the amount of overlap you can run as this generally leads to an engine that can't pass emissions.
For a race engine you'd want to increase the bore as much as possible, increase the gasflow as much as possible and damn the emissions. For a road engine a longer stroke is more advantageous, either way ultimately the peak gas flow of the head should equate to the volume of air you want to pump to achieve good combustion at the desired peak torque rpm. Accepting that your road engine will achieve peak torque at a more modest rpm means you don't have to go mad with extreme head work and expensive block work.
If by a well known Fiat engine builder you are referring to a certain gentleman in Lincoln then he will do the work for you but at a price. Asking him to just give you the specs is likely to send him fuming as he firmly believes that anyone who would do such work should know what to do anyway - the best bet is just to buy his book on the subject and then ask him detailed questions about what you read. If not the person I am thinking of then just send me a PM and I'll point you in the right direction.
In short the sort of spec you are going to end up with is a head worked to maximise and balance gas flow across all ports but with an emphasis on the inlet side. The combustion chambers may require balancing out and slight reshaping but are usually pretty good. The inlet valves need to be enlarged with fresh, high quality valve guides and seats. The head itself needs careful selection and testing to make sure it is solid - it is generally best to get hold of a selection rather that just modify the one head you have to start with and if your workshop isn't familiar with the head design you'll need an extra one to be sacrificed for analysis (chopping into bits). Details like 3 angle valve seats are the fine dressing - if the rest doesn't get done then fancy valve seats are a waste of time and effort.
You need to think about fuelling too - anything past about 90bhp is starting to push the stock carb past its limits. For accuracy, flexibility and efficiency you really need to consider fuel injection. Borrowing the Uno Turbo Mk2 inlet manifold and plenum chamber is a handy shortcut as it just bolts to the standard head plus you get suitable size injectors to start with (the Mk1 Uno Turbo head is slightly different with a matching inlet manifold). You just need to add a crank trigger, ECU, fuel pump and wiring.
Going with carbs is of course possible but it just gets more expensive and harder to keep emissions at idle under control. The difference in set up cost is minor (both are expensive - I've laid out component prices in another recent thread on this subject). The exhaust side is fairly easy to sort by comparison, a decent free-flow manifold will do the job and thanks to the short exhaust length it is pretty easy to avoid many of the problems (except noise).
