Farmall Cub

I have a cub 154 that is in need of some major motor work. My question is: I found a 184 engine for sale said to be in good running order for less than I could ever rebuild the original engine. Are the two engines interchangable?

Yes and no. The engines are nearly exactly the same, so the engine block is completely interchangable. The 184 uses a different starter, PTO clutch, and a few other things, so you may have to swap out some of these items too. I don't know the exact details, but you should look into these two at a minimum.