Its not going to be much over standard. With all the external stuff done, perhaps 15hp at the flywheel, and thats being keen.
And most of those external mods wont do much without the internals to match. eg a huge exhaust will acutally hurt performance a bit without a cam and head work to match.

Beyond headers and a free flow exhaust (in reason) anything else will add realibility rather than performance.

are carbs condsidered external?? if you changed the carb to something mild, (small webber) then a few extra ponies, but still not much.

edit : dd you beat me to the punch, and I think you hit that nail on the head.