That’s quite right, the Sirius engine can’t be reversed. Some piston valve engines can be reversed by swapping the intake and exhaust, but the engine doesn’t run nearly as well. I have an old Stuart MTB1 which has the same basic configuration as the Sirius (twin cylinder single acting with a piston valve) and I’ve just tried swapping the intake and exhaust and it doesn’t work.
In any case the Sirius will not always self start as there are only two power strokes per revolution, and even if it could be made to reverse, in a radio controlled boat, you could be left with a model out of reach and no power.
Derek, I think the 2nd para. means only castings and gears are included unlike most of the other Stuart kits that include other materials, nuts , bolts etc.
Malcolm