Check your ground wire and check if you have at least 12.5 volts at the battery. Even a battery that measures 12v may be bad. Swap batteries with your friend. When that doesn’t work, disconnect the starter from the wiring harness and check the resistance thru the starter motor. Should be a couple of ohms. Check the pig tail that goes to the harness. It could be broken or its connection to the starter could be loose. Check that the bendix isn’t stuck.
Now you’re the point for a new brain for your engine. The very old push button boxes had a replaceable fuse. If your box has a sticker that says do not open, there is no user replaceable fuse inside.
Russell Karting might be able to repair your box. Call and find out. Either a replacement push button box or the kit to upgrade to the key start is going to be expensive.
LAD Specialties customer / tony kart / rotax / kt100