I've run several engines that use the actual Dellorto carb or the SHA clone and similar mods to those you've made, not exact, but similar. So I'm on the same page if not the same sentence as you. Here's my observations on the 66cc reed setups and with the China girl motors in general. The SHA carb can be quite good but in my experience the Dellorto is much better...both can flow enough to reach 50mph when jetted properly. The carbs design limits tuning and a larger Mikuni makes more sense than trying to get a little more from the SHA carb.
I'd suggest mounting an inexpensive inductance tach/hour meter to the spark wire lead just to see where your motor is struggling. I'm guessing with the 36 tooth sprocket it's not getting much past 7,500 rpm, if that. When these engines run well they can pull over 10K and some 12K plus. Now they won't last long at these revs but they can pull to these numbers.
I normally use 44 to 48 tooth sprockets for tuning my motors, even the modified ones and at times I've found that they also hit the wall at around 7k & they shouldn't, but felt good getting there, and running 38tooth or higher ratio the motors struggled to reach even that many rpms; though I was running faster mph with the smaller sprocket I lost rpm.
Without gaining engine speed, actual speed is pretty much stuck where you are. Tuning makes the difference, but what kind and in what combination will vary. I see you've thinned the mix so you must feel confident the motor is broken in. I'd not think the motor will last very long at that mix ratio, but it will be faster. You don't mention plug chop, but if the plug looks good both fuel flow and compression can't be too far out of line. You're pleased with the way it performs all around, save reaching that elusive 50mph mark so I'd say the motor is functioning fine as also, but the performance tune isn't working...your mods are not working in combination to build power at high mid-range rpm, let alone high speed power, of which you have none at this point.
I've spent a lot of time trying to get some of my motors past this "sticking point" and each one has been trial and error, a lot of error, and no two responded to exactly the same changes made. Some wanted less or more compression, others a different x-pipe, one responded to an aftermarket CDI, jetting, carb change etc. None exactly alike. I run a third port cylinder and a window piston in two of my reed (2 petal) engines & one runs super the other not so well and both are tuned alike in every way.
On a positive note when you break through the bottleneck you won't need a tach to realize it. It's a dramatic difference when you come on pipe.
Rick C.