Of course, you can set the max output in COMMAND mode when using the PowerMaster, but I don't think you can do it in CONVENTIONAL mode. At least, I haven't seen this in the owner's manual for the Legacy PowerMaster 180 that I have. I hope I'm wrong, because--no surprise--my conventional engines can go too fast for operation by my grandchildren.
So far, I've only been able to let them run my TMCC/Legacy engines where I can set a speed limit. I'd like them to be able to run my conventional engines, but, party pooper that I am, I don't want to have a demolition derby on my layout.