Awesome. Chopped monobloc. We used to add a float bowl extension to make sure there was enough fuel in that one bowl to feed both carbs at higher revs. Yes they are still available and look more or less just like I remember.
Timing with a cigarette paper, now that brings back distant memories. I'm sure Beachcomber remembers that. OK to run a timing light can only be done if you have access to one end of the crank and in this case that means the primary drive side.
Attach a timing wheel to the crank and use a piece of stiff wire from say a wire coat hanger to make a pointer. First you fing TDC. Bets way is to remove both plugs and get one cylinder to TDC with valves closed and make a positive stop to set TDC.
That means making a rod that stops the piston at 20-30 degrees before TDC. Here's some:
https://duckduckgo.com/?t=ffab&q=Piston+positive+stop&ia=images&iax=images
I have a set of Vance and Hines set aluminum stops I bought decades ago, but I usually just hollow out a spark plug and put a bolt in it. Here's one I found on ebay but make sure that it's the same thread size as your spark plug. Plugs come in 10,12, and 14mm for bikes in 1/2" or 3/4 long threaded section. Yours will be 14mm by 3/4 inch most likely.
You slowly rotate the motor back to say 30mm BTDC (or 90 degrees) and screw in the adapter. Be gentle. The valves should both be closed so don't rotate it too far.
Slowly rotate the motor forwards and you will feel the piston contact the stop. Adjust the timing wheel and/or pointer to say 30 degrees. Just note what you set it at.
Now back the stop out and allow the motor to rotate forwards to about the say say 90 degrees or so ATDC.
Screw the stop back in and now rotate the motor gently backwards. Yes back towards TDC until the piston contacts the stop. Now you are the same distance down on the exhaust side as you were on the intake side.
Read the timing wheel and it will be quite different to the intake side. It might be 40 degrees or 20 or somewhere different. Just right down that number and adjust the wheel or pointer so that it's half way between the two numbers.
repeat and you will see the intake side will now show a similar number to what you just adjusted teh exhaust to. You may have to repeat that until both are the same.
What you now have is the same reading before and after TDC and the magic part is that when the pointer points to TDC that is exactly TDC.
Using a dial indicator or plunger is not accurate enough because of the latency at TDC.
Now hook up the timing light to its own power supply and clip the sensor around the HT lead (same side) and fire the bike up and see where it fires. You have to look straight at the wheel and pointer to avoid a parallax effect. Right down the timing either at idle or at full advance. then move the timing light clip to the other side and see where that fires. If both are identical, pour yourself a stiff drink and relax. If they are more than a degree or maybe 2 out, then the ring cam is an issue and has to be machined/tweaked to bring both back the same.
Write things down so that it's easy to repeat.
Is that all a PIA? Yes but they say the devil is in the details, but that's one reason my 500cc T100 was faster than most 650s back in teh day.. Attention to details (plus a 650 crank, slipper pistons, gas flowed head, race cams and pair of chopped monoblocs like yours.