Dave, Chuck: If it were mine, I would try to flange it. Another motor manufacturer (starts with B and ends with S) sells adapters to use the threaded holes and not the pipe thread connection. The Kohler Engines site has an accessories catalog that features several items that might be useful in the hands of a skilled fabricator.
Site Link:
Accessories
Images and Text:
EXHAUST FLANGE (CH/CV11-15)
For adapting manifold length to customer-supplied exhaust.
Part No. Description Models
12 295 01-S Steel, 0.188" thick CH/CV11-15
Edit: Part numbering didn't come through, they are numbered 1 through 5, left-to-right, top-to-bottom.
MANIFOLDS Outlet For
Part No. O.D. Models Muffler Item
12 164 03-S 1.312" CH11-15 Various 1
12 164 04-S* — CH11-15 Pepperpot 2
12 164 05-S** — CH11-15 Pepperpot 3
12 382 01-S 1.125" CH11-15 Ham Can, Top 3
12 382 02-S 1.125" CH11-15 Ham Can, Start. 4
12 382 03-S 1.125" CH11-15 Ham Can, PTO 5
* 1.0" N.P.T. external thread.
** Same as 12 382 01-S, except 1.0" N.P.T. external thread.
The other manufacturer's parts I know were hardened, and these might be as well.
P.S.: I'm still wanting to know about the Flange Assembly I posted about on the last page, if anyone has an insight to offer.