Commando gearing

Start by counting the teeth on the sprockets, you also need to work out what gearbox you have as any 5 speed is a special.

Couple of extra teeth on the front sprocket should do the trick.

My new USA export 850 Mk2 Roadster came from the factory equipped with a 19 tooth front sprocket, changing to a 21 tooth sprocket done the trick for me.

If you look at the gearing commander website you can select a commando and see exactly what changing an engine or gearbox sprocket will do. 

https://www.gearingcommander.com/

1) How do you keep the front wheel on the ground?

2) You may want to upgrade your front brake.

If the the top gear ratio is the same for both 4 and 5-speed gearing, what benefit does it actually provide? For a track bike I can see that  it would be easier to keep the engine in its peak power range, but for a road bike, would it not just result in more gear changing?  There would likely be a reduction in load on the gear train (and shell), by not having to apply so much torque for acceleration with 5 speeds, so that could be helpful.  Is there anything else that would justify the expense for a road bike?