A 50cc is about 3 hp (50/15cc) comparably in electric that would be 2200w (750x3). The bicycle being a much lighter chassis and especially wheels however can get by with less hp and maintain the same power/weight ratio.
My recent trials with a 750w (1 hp) mid drive with a low range of gearing, torque assist and a throttle that weighs less than 45 lbs. It has a power to weight ratio of .o222. A 3 hp 50cc bike that weighs 150 lbs has a power to weight ratio of .02.
Because you also are putting your pedal in put in it raises the available torque quotient. The motor I use has 80nm and it really does show when climbing steep terrain, it's like a tractor! I only use the eco setting while plonking around as it is plenty for the terrain and speed I want to travel. On smooth road you just drop some gears and can get up to speed easily even though you are on a bike with 15psi in the tires!
Having a throttle as well as torque sensing PAS is key. Also setting up your front brake lever as a "clutch", basically using the cut out when necessary by a light pull on the lever which is very useful for low speed manueuvers to keep the PAS from kicking in when not wanted. Can also be used while having to shift under load but I find just backing off on my pedal input momentarily and the gears change on the PAS runout without any stress then pedal again.