I've modified this so that it is easier to print, but it will take more time to assemble. I separated the crankshaft and pistons. Print the crank horizontally for strength. Also it makes it easier to lubricate this way. Print it on a raft as when I removed the raft it took most of the supports with it.
I've redesigned the pistons so that they snap on to the crankshaft. I've made it so that it is easier to put them on than it is to take them off. I've found it best to print the pistons horizontally. Uses internal supports on the head. It can take time to remove the supports in the head but I've found that carefully breaking the head free and leaving the supports inside works and doesn't affect performance too badly. Sand the outside of the head before you put it into the cylinder and lubricate.
I had issues with removing the supports on the main block to make the bores smooth, so I split it into 2 different sized halves. This will weaken the block but if you glue it properly it should be more than fine. I tested it comfortably at approximately 750 rpm. Print them so that the cut face is on the bottom and the cylinders are pointing diagonally upwards. Again print on a raft so that it will remove most of the supports when you peel it.
Personally, I only like to print the larger half and 4 pistons, so that I can see the internals move.
Edit.1: I've added a cut out into the block, so you see the parts move with the full block Edit.2: I've added a new version of the crankshaft. I've moved the fan to the side of the crank handle and replaced it with a rod which can be used to attach the crank to a drill. Edit 3: I've split the piston head and conrod for better printing (supports are still required. Also, I've redesigned the conrod so that it will snap on to the piston head. Remember the smaller end attached to the piston head and the larger end attaches to the crankshaft. Edit 4: I've had issues with stability with the cut-out so I've added this insert.