I have built a TVR1ABB and it was very straightforward. Lots of little parts to make up the Hackworth reverser mechanism but it went together and works very well. Instructions need to be scrutinized but they are all correct. I think a very strong engine as well. I tried to steam it from a 2" boiler but this did not work. However the 3" boiler is a fine combination and I got about a 20 minute run in an 11lb, 38" launch. I propped this launch with PropShop "Steam scale" 4-blade prop, 2.4" diameter. Have a look at the movie and see that the TVR1A on 3" boiler can step RIVAL right out. Reversing is confident and effective as well.
The design has some aluminum moving parts face to face which I don't like but the frequent lubrication makes it work ok. No galling or excess wear so far.
https://www.youtube.com/watch?v=EglzcUZKxzUI agree a search for the "best engine" is full of variables, but this one is a very good solution in my opinion.
You can buy it direct from Graham in the USA. The BB version is an option, maybe not necessary, nor the extra "for looks" special bolt kit. (but they do make it look nice. An adaptation has to be made if you want to run the reverser from front or behind the engine instead of above the engine.Suggest the Flyskyi6 radio, yes 6 channels more than you need but a good solution and computer control for a very modest price. Having "end point" adjustments will save you a lot of fiddling with the control rods. And be sure to use a metal gear servo for the rudder. Accidental bump of the rudder during handling/launching is sure to strip out a plastic gear train.
Have fun, it's a pretty boat!
-Carl