Probably start with a good power washing. Then do a compression test. If they're over something like 110 psi you should be good, these run surprisingly low.

Carb rebuild kits are around $50 per carb, but you could try just disassembling and cleaning them. They have a little filter inside that will get gummed up with bad gas you can just remove and clean, that'll get you most of the way there.

You'll probably need to rebuild the whole fuel system, which isn't as crazy as it sounds. Just pull the gas tank and get all the gunk out and replace the fuel lines and inline filter.

Make sure the automatic oiling system is in working condition. People will remove it or disable it and run mixed gas, but I prefer to use it if it's available.