The thing to watch out for on Burman gearlevers is that the 350 and 500 have different size splines,the 350 being smaller though the casting seems to be the same. Also the lever for the Comet is not straight,forget about the bends that is not what I'm refering to. If you took a standard Burman lever then the 'pedal' end is inline with the split clamp and the centre line of the splines,the one for the Comet has the 'pedal' above the centre line so that the gear indicator does not foul the lever in top gear.
If you are not bothered about the indicator the a standaerd 500cc Burman lever will fit.
Now for the best bit--350cc levers are fairly plentiful, bore out the splines to 3/4". Get a spare selector shaft and(with great difficulty) put 2 steps each about 1/16 to 1/8 inch long on the shaft.The first step to almost remove all the spline and the second to about half the spline depth, you now have a broach to form new spline in your bored out gearlever.
I only have a 2 ton flypress which was not man enough to do the job, so i used my 6 inch vice instead.
Spread the clamp end slightly(not so much that it will break) with some packing and put a bolt in to stop the clamp opening out, and then start to press the broach into the lever.Take small bites at it and remove the broach regularly to lubricate it. Once you can get it all the way through, you may have to finish from the other side, remove the packing and tighten up the clamp and run the broach through again a few times till it has reasonable splines.
This is not a very tidy solution but I have done a couple and they work well, and the levers cost me less than £5 .John