what size rear sprocket too small for 50 cc Titan engine?

for a 185 lb rider on flat surface with 26"tire, what is the smallest rear sprocket possible for a 50 cc titan engine before the engines torque is not enough to sustain top speed? Is 27T too small? what would you estimate top speed with a 27T sprocket?

I have a 2 stroke 70 cc with no modifications , 26" tires , and a 36t sprocket and run about 30 to 35 mph. little slow on the takeoff , I have to help pedal a little but on the top end I 'am moving pretty fast for a bicycle! I imagine if you installed a 27t sprocket , just a guesstimate maybe 40 to 45 mph , which should be plenty fast! the real question would be after successfully adapting a 27t , how am I going to stop?

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