Bernd,
If you mean slide valve when you talk about a D-valve, then no, you cannot reverse a slide valve just by swapping the inlet/exhaust over, it is the way the steam is fed into the chest, and requires mechanical means to swap over fwd/rev, it is done by changing the valve timing.
On the oscillator, it uses a very simple swap over plate that changes the in/ex over, while on the piston valve, it uses a rotary barrel to do the same thing.
So basically if you need to easily swap over fwd/rev, your engine will have to be either an oscillator or a piston valve.
Metal chain and sprocket final drives usually cause havoc with radio gear, producing radio spikes that are very hard to filter out. You can buy small, very strong nylon chains and sprockets that overcome the problem, but they are rather expensive.
I personally think that a 3/4" square engine will grossly overpower this small model, and so waste steam, a 1/2" bore X 3/4" stroke twin would be more than enough. But of course the choice has to be yours.
The whole top end of both engines I showed are exactly the same, and can be swapped back from one to another in about half an hour. That design of engine would be dead easy to change to the size you want as it is already 3/4" stroke. Put a 4 to 1 reduction instead of 3 to 1 and that would drive the wheels direct from a slow crawl to a fast blast. You could easily work out the top speed. Say the top speed of the engine is 1500 RPM (about normal for a low loaded steam engine), divide by 4 = 375RPM. Circumference of say a 6" wheel is for approximations sake 20". Multiply 20" by 375 = 7500 inches per minute = approx 7.1 mph (a steady jogging speed). It all depends on how fast you want to go.
BTW, using a tank for gas as you suggest is highly dangerous. In the UK they have to be made to very high standards and undergo stingent tests before they are allowed to be sold. If a small gas tank as you envisage explodes, an area of at least 20 feet across, maybe more, would be ground zero. So I would suggest a little more thought on that one.
Bogs