I have fitted a Rex RK8 ignition system to my DT 360a, it runs well.
everything is standard specs as from the factory,
however it won’t rev beyond 6100-6200 in any gear.
mixture throughout the full range is good, compression and leak down tests are also good. Gearing is as original specs.
I thought maybe Rex put a rev limit on the system?
any suggestions?

You could try removing the airbox lid to see if that helps.

Exhaust might be a tad blocked?

I think I've read that they don't rev much beyond 6k normally, although I might have that wrong. 

I have a Dt 360 that I have had over 50 years and 6000 rpms is about it.

A direct quote from the 1974 issue of Cycle magazine test of the DT360a:

"Attempting to spin the engine above 6000 rpm for more power is futile....."