You rang..?
I'm sure someone else will be able to give you a better reply (since I really don't have built a running engine yet) but I can get the ball rolling.
300Hp from a 1.8 as old as this is quite an extreme task. That is a fact, whatever people say 300
real Hp is a lot, I know you understand this Andrew but I'm just setting the mood here.
I have heard tales about how bulletproof the bottom end is and I have also heard about how fragile it is. Some say you need a monoliner if you go over this many Hp and some say you can use standard rods for one gazillion Hp.
For reliable Hp the
rods need to go and most certainly the
cast pistons. Forged items is a must and the lighter the better for reliability.
Oil squirters is also a must if you are going to use the engine for anything other then dyno runs (or freeway squirts). I asked friend who works with turbocharging at a Swedish car manufacturer a couple of years ago if oil squirters were needed and he lookt at me a bit funny and then said "Is an intercooler needed?"
.
You may not need it for the application but it is really just stupid not to add it, saves pistons. High capacity
oil pump,
windage tray and a
modified sump helps keeping the engine happy. Basic stuff really. Bin the
dizzy
Head needs some smoothing and new
valves and
guides, go for 45 degree seats while you are at it. Get valves with thinner shafts, lighter and better for flow.
Valve springs of course to match a high lift cam.
New
cams obviously as the standard units at total crap, I have no idea what the Alfa engineers were thinking when they designed that discusting exhaust cam but I'm sure it wasn't maximum power. New
plugs!
That pretty much takes care of the long block, you might want to add
O-ringed liners if you wanna be absolutely sure and possibly even a litte engine "
concrete" (epoxi really) to keep the liners in place.
The
manifold you need to replace and naturally the
turbo, I'd prefer to run relatively low static compression and a well flowing turbine to keep the temperatures in check, less backpressure is also nice. Matching exhaust, 3" or something will probably do the job just fine.
New
intercooler/intake, big front mounted air/air intercooler and a well designed
plenum that has a nice and large throttlebody and velocity stacks on the runners.
Injectors need replacing too as the standard ones are VERY small.
Fuel pump needs to be replaced and possibly the
fuel regulator
Management system should naturally be able to control both ignition and fuel, I don't have to tell you what to do with the LE2-Jetronic I hope?
Waste fire or direct ignition, either way should work. 60-2 missing or similar high definition trigger.
Coils and sensors to match and someone to hook it up and tune it.
EGT measurement (induvidually at the start of each branch by the head and one in the collector just before the turbine) and
wideband O2 measuring systems is a must for serious mapping.
I guess this is just a BOM-list but it gives you a rough idea what a race engine needs...
A street engine pumping 300Hp might be easier as you tend to not go as hard on the street (or turn as much) and if you do you are probably going faster = better cooling.
Heat is aleays an issue in turbo cars and the more junk you can move out of the engine bay the better, a new radiator might be a good idea too.
As for specific designs of the individual parts it is really down to what kind of system you build and you need to give some input before I can give you any useful answers.