Bonneville Chain Drive

Nearly all modern motorcycles have the clutch on the righthand side these days due to gearbox type/design used and with the 900 and 1200 engines fitting the drive on the right was the simplest engineering option for Triumph as they fitted the clutch on the left to keep the twin engine with its caricaturist primary case on the left and timing cover on the right cleverly shaping the front sprocket cover to look like the original gearbox outer cover.

 

I'm assuming it's also easier to keep the gearbox on the left side due to the foot change lever being there, rather than the large rod all the way through the engine as on the 1975 and onwards T140 engines. Swap the gearbox side, then you'll need to swap the clutch side or the engine will stick out a bit on the left.

As a side note us Britishers had the gear change lever on the right in order to have the brake lever on the left, that is, to keep it away from any sidecar that was fitted. Due to everyone else driving on the wrong side of the road (how could you draw your sabre with the right hand and cut the enemy's head off if you pass each other on the left - just a silly idea.) with the Japs (who still keep to the correct side of the road because they are true warriors, like the Brits) and us wanting the dollar we changed sides due to the US's sidecars being more important than ours.
Moral of the story is - be careful when getting on a pre 75 British lump of Iron and watch out for your feet if you have a sidecar and keep to the left (correct) side of the road