You will indeed have to get new lifters with a new camshaft. As for the 100 HP I think you can come pretty close, perhaps 92-95...ish, the one aspect we don't talk about on these threads you up the horsepower on A-series it ups the RPM range and at a certain point the gearbox can no longer deal with the narrowed power band. I like the suggestions so far, GX carbs & Cam, 10-1 compression 13-14lb flywheel and port work.

Tom

you don't need new lifters, you get the lifters you have refaced so that they match whatever cam you get. its about $5 per lifter

as for getting 100hp, thats about 75kw. If you're talking at the wheels i'd say no chance.

a15 is 80hp at the engine so an extra 20hp is possible but youll
need rpms and bigger valves also unless you want to rev sky high.
Its going to be hard unless your camshaft is a little more serious.
Also if you are going to rebuild her with a late a15 crank,
77.5mm slugs and total seal rings once you up the comp to 10:1.
Maybe 44mm sus will also help compared to 38mm hitachis.

Otherwise simple ST turbo with SU for 100atw at halve the cost.