Norton 350 featherbed framed model 50

......and start your kick just after TDC on compression, the single is way easier. Compression every other revolution rather than every time around.   All technique.

...and the 350 has a decompressor to allow you to take it up to TDC before giving it a prod.  With a lower compression ratio than 650SS it should be much easier to start. The mystery to me is why the big twins had no decompressor.  Possibly because the 88 was first in line at only 250cc per cylinder?

It's also a good idea to retard the ignition.  The technique on my ES2 (500 single) is to retard the ignition a bit, tickle the carb a bit, use the decompressor to ease the kickstart just past compression, let the kickstart return to the top and give it a steady push all the way down.  As soon as she fires, advance the ignition and that's it.  It's a lot easier to do than to write.

Good luck putting valve lifters on a twin cylinder bike. 
I wouldn't say it's not possible or has never been done, but I've never read of it being done - and successfully! 

Errr Michael, the original question was about the comparative ease of starting between a 650ss and a Model 50 (single cylinder).

Roger, my 1960 Slimline M50 is so easy to start I feel I could start it by hand! I have removed the valve lifter, as it is quite easy to ease over TDC, and then just a gentle kick and away she goes, to a very steady and slow tickover when warm. I wind it through the gears, and use it as it was probably not intended, but it is great to ride, and is my bike of choice most times. Not as quick obviously as my Venom Clubman, but nonetheless a great bike.            

Regards John O

Before 1959 they had magnetos. So a decompressor was the only way to stop the engine unless you stalled it in gear.  The magnetos used with the twins have an earth terminal cutout (unlike the singles) so don't need a decompressor to stop.  And it saved cost to leave out the complications.  There's been previous discussion here on fitting a chain saw type decompressor to a Commando.  No idea if it's been done.