Trango could probably give you his specifics, but I'll shoot from the hip and give you a few points to ponder. First off, what is the safe max operating RPM of the engine you want to install? If for example you pick an engine with a normal operating speed of say 3000-3200 RPM you instantly gain 25-33% more top speed vs the rated 2600 RPM for a LDT-465 (assuming the "new" engine's HP/torque curves equal or exceed the LDT-465).
If the "new" engine operates in an RPM range closer to that of the LDT-465, your best option for speed increase is larger tires. That option is a lot cheaper than custom gearing, although you might be able to find a tranny that you could adapt with a higher OD ratio, say .75-.70 vs .79 for the Spicer 3053A.
MPG is trickey. Horse power takes BTUs, BTUs come from fuel. More HP equals more BTUs equals more fuel. However, some engines are more efficient than others, and fuel efficiency varies over the engine's operating range. You would have to compare the specific fuel consumption curves for various engines to see what the relative HP/specific fuel consumption characterictics are, and even that will only give you an idea of the direction your MPG is going. I do recall seeing the specific fuel consumption chart for the LDT-465, as I recall the "sweet spot" was around 2000 RPM. Below or above 2000 RPM, MPG decreases per unit of HP.
So if you were to examine the HP/Torque/Specific fuel consumption charts for a whole bunch of engines, pick one with a normal operating RPM in the 3000-3200 range, HP/Torque curves better than the LDT-465 and good specific fuel consumption in the 2600 RPM range, in theory you could have higher top speed, more power and better MPG with your repower.
Chances are that isn't going to happen though. You're going to be stuck with a very few engines which you can find which will fit in the Deuce which you can afford to buy and go with that. I would suspect though that most any recent commercial diesel you can find that will fit will be an improvement over the LDT-465. I'm not knocking the LDT-465, it is a great engine and pretty bullet proof, but it IS 40 year old technology, and there have been a lot of improvements in engine technology over the last 40 years.
