CB or Berg for the merge header- but keep in mind that a merge header puts the collector pointing straight back, about even with the rear valance. Any kind of exhaust and you are hanging out the back. An A1 Sidewinder puts the exhaust on a right angle, so that everything fits, but it's expensive and you'll probably need to drop the muffler to adjust the valves. There is no cheap/elegant solution.
You're also right to worry that finding a head porter who knows what he's doing who would be willing to work on used heads (that he didn't sell). Unless all you are after is a "fluff 'n buff" (which won't really make the engine flow any better), I think you'd have better luck finding a particular needle in a stack of identical needles. Good head guys are backed up for 6 months or more selling completed heads for their own particular program (Stage 1, Stage 2, etc.). If you could find somebody to do it, I'd wonder why- and even then, he'd want your heads for months and you'd be at the bottom of the pile.
Panchitos are $700/set with the CNCed chambers and dual springs. You'd be pressed to replicate that with somebody else.
We haven't talked about your crankshaft or cam.
... which gets us to why I'd just start over. I can almost guarantee that you have a stock, non-counterweighted crank, stock rods, and a stock cam. To turn more than 5000 RPM safely, you'll want at least a stock stroke counterweighted crank. If you are doing that, there's zero reason not to at least stroke it to 78 mm. With the case split, you'll want a different cam.
There's a sweet spot with these engines, and it's at 82 mm of stroke, stock length connecting rods, and "B" pistons. Do 90.5s, and you've got the 2110 that everybody loves. Use 94s and you have a 2276. A W120 works well with either, an FK8 works great with the bigger engine. You'll want a 1-5/8 header with either (although a 1-1/2 would work fine for the 2110) - but it doesn't matter since you're looking at a new exhaust anyhow.
... and you can see how just building something new makes more sense the longer you look at it. Alternately, you can fritter around trying to make a silk purse out of a sow's ear, as I've done repeatedly.
Or maybe you're like Jim Ignacio or Mitch Toll who have embraced the limitations of their cars, and learned to love them for their imperfections (or something).
I'm not that guy. As you are asking for more of your VS 1915, I don't think you are either. Everybody tries to find the middle road, to thread the needle and come up with a powerful engine on the cheap. It isn't out there, and I've tried. Don't be like Stupid Stan.
Buy once, cry once.