Is it possible to change to a larger and type of crankset from a one-piece Ashtabula style? I made an E-Bike out of a cheap bike. It uses those one piece cranks. I was wondering if I could get a bigger gear. The motor top speed is 36km/h and I would like to assist the motor while going at that speed. But as is, past 24km/h, it's too fast to allow me to assist.
With this types of crankset I never found anything different than that size gear. If I found some other crankset types would it fit into the frame's crank shaft? Is it standardized?
My bike has a 5 speed derailleur, 20inch wheels. The crank is as seen in the example bike image. Motor is front hub.
One piece cranks are used on inexpensive adult bikes so larger chainrings should be available as replacement parts. The crank-chainring interface is standardized. See Sheldon Brown's page on one piece cranks.

Note that the size of the chainring will be limited by the clearance between it and the drive side chainstay.