I've been pondering this, and have concluded that, since Rudy and Birk Petersen (whose "Live Steam" construction series I have) both used what looks like the same model of steam roller as inspiration, I can use both of their models in designing my version of Rudy's roller.
In determining overall dimensions, I have decided to make something 60% of the size of the Petersen roller, not least because the channel frame rails I have just cut come from a piece of 3/4" square aluminum tube that I have lying around here. Of course, that's not the only consideration: I have a nice copper tube of about the right size for the boiler, and the overall length of Rudy's model seems to be about 60% of the Petersen roller's length.
The finished model will be more Kouhoupt than Petersen. It all looks feasible right now, and I'll plod along this winter.
My most serious concern, though, is the engine. I can see little of Rudy's engine in the photos I have, and the Petersen engine looks to be beyond my capabilities, so if anyone can suggest a rather basic little twin-cylinder horizontal slide-valve engine I'd appreciate a hand here; otherwise, I may just try something of my own devising....