interchangeable parts

[rusty hook]

I have a 97 mercury 40hp, 2 cylinder, 2 stroke. A ground wire broke (again) and fried some electronics. My mechanic said all he can do is throw parts at it. I found a 25hp 2cyl 2sk with a blown engine for $300. Good electronics AND a good lower unit (mines not so good). Are these parts interchangeable?? Logic tells me yes but brain power not my strong suit.