Rich,
I have a PA 51 which I took out of my Geo Juno (Saturn) to replace it with a PA 61. Why? I dunno! LOL!! But it IS sweet with the .61. I can use the muffler or pipe on the PA .51. I was going to build another USA-1 (and will someday soon!) but I have a beautiful set of foam cores for the Genesis 46. I *could* also use a PA 61! And I have a great running T&L ST G-51.......
The only "big" OS engines I have are 40 VFs.......... of course, one of those on a pipe would definitely work! Would like to stay with a muffler on this one, though.
Jim, I have zero experience with the Stalkers except for a 46 which was in the first run they made.
Steve, I don't have a DS 54, and would like to use something I have in the drawer.
Charlie Chan, I am *thinking about that one...... weight of the engines I have vice the ST 46 in the original.
Ward, see above!

More ideas?