Interesting thread here.  Question for the group. 

1970 RT1… can one use.. say.. the 71 RT1 decompression ported cylinder head on the 70 with that timing advance feature? 

Drac

Layman's guess here , but I don't see why not.

The decompression valve is manually operated (unlike the 1974 on models), so doesn't have to be used.

Worse case scenario on using both ignition retard and decompression valve is that the engine won't start.

I await the educated experts' replies!

Photo of the head shows a standard compression head so i guess your cylinder is just a trail one as well.
Then again on cmsnl it doesn't show one on the "71 MX but does on trail models "70 to "73 so i don't know anymore.
CMSNL show a decomp with another diagram of the cylinder down the page of the trail models.
www.cmsnl.com/yamaha-rt1_model16783/
I had a piston port cylinder with a decomp valve plus the retarding flywheel, but was a bit of a mix n match motor.
Could probably have started it by hand as was low on comp even before the decomp was open.
Kick Started So easy. Should have 351 on the base of the cylinder.
photos.app.goo.gl/TCCZ9py6j4N3spQ87