Oh man that was some thread that
I'm hoping to get the flag carrier (as it is now known, can't think why) out and sorted for June for the wee run down to Derbyshire...
Anyway, back on topic,
Without taking the engine to bits concentrate on breathing, 4 branch manifold, larger exhuast but I would suggest keeping the cat on, robs the engine of torque otherwise...
Taking the engine to bit, improvements can be made without spending loads of money, fetch the head off and get all of the valves out of it, porting and polishing is pretty easy, a die grinder, some measuring calipers and away you go, loads of stuff on the net about porting, matching the inlet manifold is also dead easy.
Next thing, no specific horsepower gain from this as such but getting the crank, rods, pistons balanced makes a huge difference to the way the engine spins up.
Do the clutch and fly as well but look to lighten the fly, losing a kilo from the bottom end &fly has the same effect as something like a 12 hp increase in 1st gear, 8 hp in 2nd and so on.
On the inlet side I tried a couple of different things, bmc cda but they aren't cheap, next best option IMO is k&n panel in the standard air box....
All that said the best single upgrade on my 155 was fitting big anti roll bars.
I think the key thing is take your time considering what to upgrade, I collected all the bits for mine over about a 5 year period and ended up with a pretty well rounded package which I think adds up to more than the sum of the parts but then I would say that