Have your dad get a manual reel mower for your own lawn. Then use that for mowing the neighborhood lawns. Reel mowers are quiet and cut the grass evenly with much less risk of scalping the lawn. Get a grass catcher basket for the back or you can leave the clippings on the lawn as mulch. The neighborhood will appreciate the lack of noise and that can be a selling point for your service. The manual reel mowers are easy to push and for suburban sized lawns they are as fast as using a gas powered mower. And they’re safer than gas powered mowers.