Yeah, it's very close. Just a couple small things different. The higher gain signal path is pretty much the BE circuit Dave uses in most of his amps, including the Smallbox and Pink Taco. Not sure what's in the runt.
I know from your other threads that you're not knocked out by the Runt 20 you just picked up.
I'd maybe suggest trying a Pink Taco, and see what you think of that, before you send the Runt in for modding. It is also based on the BE circuit, but it will probably be a little rawer and brighter than the Runt. If that does the trick, you'll save yourself some cash.
However, a 20watt smallbox would be a very cool amp. The Custom Shop Smallbox 20 pics I've seen look like a modded Runt, so I guess if you send your runt to Dave, that's basically what you'd end up with.
Either way, I think you're better off working with Friedman than trying to build an amp from scratch, especially if it will be your first build.