Most bike forks are either 1-1/8" or 1" diameter, and in your case I see they are threaded forks.

Looking at your bike picture, it seems like this would work, Am guessing that your bike is 1", but they also sell the bigger diameter. The main question is whether the steerer threads are the right length to match your steerer tube. You have to report the length of your current tube, as well as its diameter,.

Representative Fork for discussion,
https://www.modernbike.com/product-2...xoC1XYQAvD_BwE

This is a steel fork, but you will also need a pair of torque arms to prevent the motor from spinning out.

Torque Arms: I have used this guys products in trhe past, including the torque arms. THere are better quality available from other sellers, but they worked for my smaller motors.

https://www.amazon.com/EBIKELING-Uni...T=1&th=1&psc=1

I've often wondered if people should even ride these trikes above 12 mph. Some owners have told me they will tip over at speed. The brakes are weak too,