Cummins makes a wide selection of engines. Most popular are the 6BT and 4BT engines. Earlier 12 valve 6BT engines are mechanical, meaning they don't require computer control. Same for the 4BT, which is nothing more than a 4 cylinder version of the 6 cylinder big brother.
I applaud the idea. I caution the path, though. NOTHING will bolt in. You will need fabrication skills in order to deal with crossmember/oil pan issues, motor mounts, transmission mounts.... which is another bag of tricks.... you will need to find a NON ELECTRONIC transmission that will mate to the Cummins. If you've already got a manual car, then you will either need an adapter kit (Advance Adapters, etc), or a transmission already meant for the Cummins. Bear in mind, they are truck engines and those use truck transmissions, generally.
Where can you find one?
Cummins supplied "light duty" 6BT engines in Dodge trucks since the late 80's.
The 4BT can be found in older delivery trucks... the local bread distributors, FedEx, etc... all trucks before the mid-2000s.
OH. If you have money flowing out of every orifice of your body and can't think of anything better to do with it, Cummins offers the newest version of their 4BT brand spankin new. Look up Cummins Repower.
Good luck.