For ethanol, you need alcohol... this might make it too obvious, but you need to use the coin to get the needed item.

For Hoagie aside from the bowling balls, you need 2 other things... you need to finish Laverne's path in order to get one of those things, and the other is a bit tucked away (I didn't find it until I realized the game has an option to show clickable areas if you press the spacebar). He won't assemble the device until... You also need a suitable place to use it.

Don't think about it too hard... let's just say other people have had the same problem.

You clean the coin very VERY close to where you actually end up using it.

If you can't find it and it's driving you crazy, here's a walkthrough: