As well as my new Grubee GT5A, I have an older 66cc engine, its ports were worse than the new bike in most respects. More casting and plating slag in the ports, and conservative port timing. It had less top speed and hill climbing power than the newer motor. As well, its crankshaft balance was worse too, so that over 25mph it vibrated terribly.
New, the Grubee GT5A was capable of 20-25mph (35-40kph) as it came out of the box and had no undue vibrations in this range. The speeds picked up as it it broke in through the first couple tanks of fuel to 25-30mph (40-50kph), I also started using less oil in the fuel mix too, which seemed to help, 20:1 to 40:1. I might recommend using 32:1 from some things I've seen now. I used varying power and rpm to break in the engine, using full throttle about 20% of the time in 10-30 second bursts in the first 30 miles.
After break-in I started doing some hill climbs to the limit of what the engine could stand and heard a "popcorn popping sound" rattle coming from the engine. This was detonation and eventually it blew a headgasket and likely warped the head. The squish area of the head was poorly shaped, so I sanded the head flat, and reshaped the squish area to match the piston with sandpaper on the head of a piston, and set the gap at 1mm (0.038"). This was a huge benefit, stopped all detonation rattle and overheating of the engine.
All this was with the fuel mixture decidedly rich. With the head improved, I leaned out the carb by lowering the float level and dropping the needle and got a much more lively engine and speeds in the 50-59kph (30-35mph) range. Port matching the exhaust pipe helped broaden the torque, but port matching the intake made things worse so I went back to a stock intake Z-offset manifold. This is likely because the mismatch blocks reversion pulses. At this point I am running 54-59kph quite reliably, rarely and barely breaking 60kph.
I pulled my cylinder to try reshaping the transfers, only to discover the stock filter had let dirt past and scored my cylinder. This is when I increased my oil to 32:1 and gained some speed and power. I tried a better foam in the filter but it reduced speed and power. I tried with no filter box and was able to break 60kph (37mph) even with the scored cylinder.
My life history, sorry for the book, hope it helps. Speeds are Garmin 76 GPS supplied on a level stretch of private road and can vary as much as 4kph from day to day. Tire pressure, lube, brake drag, temperature, humidity and wind can affect the speeds so I try to do any testing all in the same day, within an hour of each other if I can. It feels like 60kph is the limit for rpm of the engine, probably limited by carb, port timing, crank balance, exhaust pipe and the 44t sprocket. The bike is a heavy framed mountain bike with 26"x2.3" tires.
Hope there is something in here that helps.
Steve