For both Simon and Elspeth Beard ("Lone Rider") who made a 2-year journey on a /5? BMW ten years after Simon, much of the trip was what we Westerners would consider, off-road. The bikes took a terrible thrashing, as did the riders, which in Simon's case included being jailed by a couple of huntas in South America. Beard pich-poled her BMW in a sand wallow in the Oz outback and was seriously injured. Her traveling companions patched the bike together while she was in hospital. As their accounts made clear, the simpler, the better.
A Commando is simple compared to modern, electronic-laden bikes but perhaps not simple enough. Brazed-lug frames can be fixed in Timbuktu, mechanical brake cables fabbed in Fez but could one have gotten isolastic rubbers or PTFE washers in Istanbul? My own experience with Commandos is that even when properly looked after, they are a bit delicate. I've ridden a 500 Triumph twin and it's not too far removed from a two-wheeled tractor, yet still too far for such tortuous journeys. When you see pictures of Beard's bike after a year or so you can see items she forgot, like a stone guard for the headlight. Simon had to have new pistons fitted for the first time in South Africa only a few months into the journey. Elspeth broke a fork tube and later had to replace individual diodes in the alternator's diode board, which was unavalable locally.
What British bikes of Simon's era would have been suitable? Norton no longer made ES2s or Dommies. Matchless was long gone. Maybe a 500 Enfield Bullet? Were they making those in India by '73. Were there suitably large Japanese bikes in 73? The little Hondas were ubiquitous across Aisa. Parts would have been no problem but aside from having seen father and a couple of kids on a Honda 90 with a dozen live chickens dangling from the bars, I don't think that would cut it in the Outback, except maybe in the Steakhouse parking lot. I think Simon and Beard picked the best of the lot for their respective times. A commando might have been more comfortable and faster but think of the reliability upgrades owners have developed since 1973. What would you have done had a lay shaft bearing cratered in Katmandu? Or the cam gone flat in Pnom Pen?