Holeshot.
With Lobsters low cost carb improvements, on what i presume to be an adapter on the stock manifold, then i think that a 2" exhaust with low restristion muffler[s] might be about as much as it would need in this state of tune, keeping in mind the stated criteria of not wanting to replace a lot of parts, or spend a lot of money. From memory, the last time i priced new extractors & turbo muflers, they were a bit pricey.
I wonder if a stock A14, or A15 exhaust manifold would be an improvement. They would certainly be cheap enough as most would be glad to get rid of the ones they have got.
The only cost then would be a new manifold to manifold gasket & some new bolts to join them together & new flange studs & nuts.. [i always use new bolts & studs] That is, assuming that it will bolt to the A12 inlet, & i suspect that it will.

That exhaust of yours is good, but methinks a little bit too good.[$$$]
Chris