AK, I wish I would have seen your post months ago...you probably have already repowered your machines. I also have an 1816, and bought a 16hp Vanguard from Smallenginewarehouse.com.
I got the engine fairly quickly, but didn't measure the shaft right before ordering, and got one smaller than I needed. It took some careful welding, but I made it work.
Anyway, to put it in, I also had to weld together some motor mounts...more so to raise the engine slightly so the pulleys would match up better. I did get everything together, then the pulley welding I did broke. I found out the pulley was cast iron, and had to weld it differently using nickel rod. It has worked since then, and it's a great little machine now, and uses much less gas than the previous engine did.
One problem I had was trying to get the muffler fit in. Smallenginewarehouse does not sell mufflers with the engines, you have to buy them from them separately. I did buy it, since you need the header for the engine you buy. I bought a different muffler cheap, and it sticks straight out the back now.
If you would like, I can email pictures of what I had to do to get it to work...although it didn't take much.
Are you willing to sell any parts off the other machine? I would be interested in a drive pump.