I replaced an engine on riding mower with another one. The original engine was a Brigs 10hp. I have a 12hp brigs given to me. But it is from an older mower. The 12hp engine only has two wires coming out of it. The 10hp one had three. There is a seat kill switch on this mower, but the engine now going in it may not have had that system. How do I wire this up now? And what do I do with the yellow wire left over? Thanks

That sounds like a much older engine of there is no kill switch. You should be able to abandon the kill switch wire and just put a thumb nut on it.