I think you could do an inline setup fairly cheaply. You'd lop off the transmission from one case, run chain from one crank to the other, probably on the alternator side, then hook up some hall effect sensors for ignition. If you can do the work yourself and have the parts on hand it could be done dirt cheap. It would be heavy, under powered, but extremely cool.
I'm doing an inline 3 cylinder right now. Three cranks, two turning one way, the other turning backwards (gear driven obviously). They will be at 120 degrees firing order, the cylinders are horizontally opposed, two on one side with one on the other. Unfortunately I have to move next month so work ground to a halt about a month ago. I have wax ready to do test runs on the crank case. I own 6 motors that I will be using the cylinders, cranks, rods, pistons, reed cages, and carburetors from. I will machine the cases myself. I already have the tooling. It will cost around $125 for the raw aluminum for the cases. I have $40 in wax to do the test runs. Of course that uses $6k worth of engine parts from a decade ago, but they are valueless now, so why not put them to use? So if you have a pile of RD parts floating around I say go for it.