lwt me see.. you want to "upgrade" a 7 speed bike with the gear ratio 37 - 73 currently to a one speed bike with an higher top speed ? Like 80 isch or maybe even more ?
You will need a ramp to get started, and forget any uphills....
Besides its not wise to mash the gears on a folder, rather spin sitting down with a higher cadence, which is better for the knees and the folder as well... Dont want to hear down the road..OOOOHHH all those folders are sooooo flexible ...
Best Thor
p.s. generally speaking the prefered gear ratio for a folder one speed is in the lower 60's