Hi Daimon,
If it has been sitting in the dry, and it was running before it was left there, there should be no problem in starting it. I'm happy to give instructions, but first we need to know what the drive arrangement is. Because it has a side-car, it will likely (but not necessarily) have a clutch and gears of some kind. Later Rovers can have a gearbox (chain from engine to gearbox, belt to the back wheel), but in 1912 it would more likely have a clutch and gearbox either (a) in the drive pulley on the engine shaft or (b) inside the back hub. If (a), it probably has a little tram handle control, or (b) will have a foot pedal clutch and a lever to select the gears (if there are any). If it's something else, you might have to post a picture. Let us know.
Leon