a couple reasons not to use a junkyard crankshaft: a single-plane crank, with a single-plane cylinder 'block' denys the the possibility of self-starting. Too, you'd have to look a long time to find a 4" stroke 4 cylinder engine. . . maybe a diesel crank out of a piece of construction equipment, but that won't be cheap. (or light)
having a two-plane crank as shown here allows the engine to self-start without user intervention. you could step up to a big-block V-8 chevy crank and get the phasing right for self-start, but you might still have to mess with regrinding to get the right stroke. Plus that's a LOT of mass for a 1hp engine.
on the flip side: People who build model engines grind/build up cranks quite frequently, and rarely have problems, even at 10,000 rpm. at 1 HP, 1500 rpm, I don't forsee any show-stoppers, but short dowel pins to keep the webs from fretting could be a reliablity refinement. (would also make phasing the engine easier?)
on to the questions:
So I'm looking at this, and thinking that you're likely to have a nasty rocking couple on this thing. Not that it can't be done; look the flogging NASCAR gives a similar crank. . . a little bit of counterbalance helps a bunch.
I'm just curious why you chose that form over one with better primary balance. . . any reason not to make adjacent pairs 180' out from each other; maybe go down to a 3-main crank with a double-length web beween the two throws; and change planes across the center main? true, you'll still have some primary imbalance and some secondary couples, too, but it seems like the magnetude would be smaller?
another question: are you planning any sort of cross-head to keep from wearing the packing glands out with side thrust?
just thinking out loud. . .
(can't wait until my milling machine arrives. . .)
-Dan