That early 70's Commercial 12 most likely has a single cylinder Kohler on it. Those engines are solid will run forever. Any reputable mower shop should be able to work on it for you. But, if you want to do it yourself (which I'd encourage!), I would start first by checking the basics. Does it have spark? (You can buy a spark tester from Harbor Freight or Amazon fairly cheap. It's a handy tool to have). Is it getting gas to the plug? If it's been sitting for a while you will most likely need to clean the carb if you didn't drain the gas and/or run the carb dry before parking it. Also, there's probably a valve under the tank for fuel shutoff. If you're not getting gas to the plug, make sure the shutoff is open.
If you don't have spark, again, start with the basics. Replace the plug. Open the points cover and make sure the points are opening and closing as you crank the engine. You might need to replace the points and condenser. Point gap is .020 if I remember correctly.
Also, look for any evidence of mice getting up under the cowling. For some reason, mice seem to like to build nests under the cowling - which could also mean chewed wires under there.
Another possibility for no spark could be malfunctioning safety switches. I don't know what year Gravely started putting safety switches in the handles of their Pro machines, but if you have them, check to make sure the switches are opening and closing as they should. Pictures of your machine will help to determine if you have these safety switches.
If you're not getting gas to the plug, you will probably want to have the carb rebuilt. Before you do that, you can take the air cleaner off the carb and place your hand over the intake to block it. When you crank the engine, you should be able to feel the suction against your fingers. Essentially you're choking the engine in order to help force fuel up to the cylinder. A little bit of starter fluid will sometimes help as well. If you do need the carb rebuilt, you can probably have a local mower shop do it for you for not much money. You can just take them the carb and not the whole tractor. Make sure you provide them with the model and serial number from the engine.
Once things are running, be sure to check the oil level in the transmission. It uses straight 30 weight oil. Don't let anyone tell you to put 90 weight gear oil in it!
There are also a couple of Gravely groups on Facebook. If you're interested in those, let me know and I'll give you the names of them so you can look them up.
So sorry to hear of your brother's passing. He did leave you a great machine. I have three Gravely walk behinds that I put a lot of hours on. Be careful with yours though - those older machines would not be "OSHA Approved" in today's world.
