I forgot on an M20 looking down the spark plug hole only allows you to see the intake valve. So bad advice on my part for this particular bike! If it was overhead valve, it would work.

There is no inlet track inside the head so looking down the plug hole with a colour tune works just fine & dandy.
I do it quite often but of course you have to run the engine from an alternative tank beacuse there is not enough space between the engine & fuel tank
Other problem is the Colortune was never meant to be used on a cast iron head of an air cooled engine so don't leave it there too long unless you want a plastic coated head .