I ordered enough parts to make a dummy ATC250R engine, and waddayaknow, it fits. I did have to cut off a couple of engine mount tabs, but the concept still appears viable; I haven’t come up against any real showstoppers so far.
It’s not wedged in, but there isn’t much wiggle room: I cannot move it up, down, or back significantly without swingarm clearance and sprocket-swingarm alignment issues. I cannot move it forward much, either. The counter-balancer drive is under the left-side engine cover, so it extends further out on that side, hitting the downtube. I can trim off what remains of the old mounting bracket, which will give me room for the missing screw head.
Amazingly, the Benelli rear motor tabs were exactly the same width as the 250R engine’s upper rear mounting boss. The bottom bolt hole even almost lines up!
And here’s the view from the left side, this time with the tank slapped on top.
It will probably get Ninja 250 forks. Not sure about wheels yet. The vertical cylinder reminds me of an old Villiers 2-stroke. I am normally a fan of cast wheels, but perhaps this bike needs wire spokes.
Side note: My cylinder arrived damaged because the seller tossed it in a cardboard box with one layer of soft 1/4″ foam. This is the second Ebay cylinder fin I’ve had damaged in transit from inadequate packing. Fortunately, this can probably be easily be shaped and filed without the end result looking too drastically out of place. Of course, I might buy a complete engine at some point, and end up using this cylinder only for mock-up purposes.