"You'll find your 90cc Honda motor will put out more power/speed (though not torque) than a cheap 125cc Chinese motor will"
I have a CT90, got it with a good running original motor, good compression, nice clean carb. With my butt on it I could see 45mph under ideal level conditions, a lot less if wind or an incline was against me.
I bought a Pitster Pro 125 with electric start and a 12v electrical system, took my time making the conversion as clean as possible (redid the wiring looms, new right-hand switch gear so the kill works properly, etc) and it's massively more powerful, starts even easier, idles better, holds 50mph in all but the worst adversity and runs at lower RPM doing so (17/45 instead of the 14/45 that the Honda had)
I had to give up the low range gear box but the added torque from the 125 pretty handily makes up for that. And I still have an altitude compensation switch on my new carb
Added bonus, headlight's a lot brighter and doesn't go dim at idle, speedo light is brighter (all lights were, of course, replaced with 12v units)