I have a DT360a, original bore, with the standard Rex ignition kit fitted. It won’t rev past 6150 in any gear like it’s been speed limited.
I messaged Rex, they replaced it with a modified kit. Then it wouldn’t rev past 4200 and started to splutter.,won’t go any faster. Carb is good and set up with good mixture readings.
refitted the first Rex kit, back to 6150.
Any ideas? Crank seals are good, compression is 145 psi.
Tacho and Speedometer are NOS and working fine.

They're not a high reving motor but any lack of top end performance often ends up being exhaust related. Even when it's limit dropped way down to 4 something you said. Could be exhaust. To explain. I had a little yamaha AG100 farm bike which was blocking up in the tail muffler so pulled the baffle & shook all the loose stuff out which came out looking like round balls of carbon. These balls would find their way into the baffle if I hit a decent bump at speed. So ended up giving it a good burn out then nothing left to find its way in to block it.photos.app.goo.gl/SCABAiEubXwHYA5g9

Thanks for the suggestion, but the exhaust is also NOS, no obstructions.

I'm not a fan of the problems Rex systems have sometimes and the way they take care of it.  If you're in the UK, at least they seem to try a little if you get a bad one.  

The kit that only revs to 4200 obviously has a problem if the other kit goes to 6150 on the same bike without any other changes.  Really hard to know if your problem is their ignition, but I understand that it really seems that way!  Some people say their systems are great, others can't get them to work.  

That said, I agree with RT325 about if not being a high-revving engine.  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....."

How does it rev without a load?  (Also make sure it's not a gauge issue where the tach stops at 6150, not likely but can happen)