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It'd be easier to convert a LA 46 to a LA 40 . 

or just find a past it leaky .46 & refit it .

Motor,

According to OS, the cranks and the rods are the same for the 40 and 46 LAs. These actually came out of the 35/40 FP engines. I got this info from the OS website:

     https://www.osengines.com/parts/xr13390.html

That explains why the 46 vibrates more than the 40. Bigger, presumably heavier piston but same counterweight.... Might be time to lighten both the piston and the rod on my 46.

I don't know the answer to your head bolt pattern question.

Dave

PS--Ok, I was curious about this topic, so I lined up two OS-powered profiles and here's what I can see--the head bolt patterns are not the same. I didn't take apart either engine to get accurate dimensions. I just used a caliper to eyeball the centers of the screw heads. It is a trapezoidal pattern. The lateral spacing of the forward bolts looks the same. The lateral spacing of the aft bolts looks the same. The fore and aft spacing on the 46 is significantly bigger. I thought the head casting might be the same. The fore/aft dimension on the 46 was only about .015" larger. It does not answer the question you really want to know, which is how much meat is there between the case bore and the existing screws on the 40 case. You might end up cutting into the gasket/seal area, which by itself isn't impossible. You might also have to notch the flange on the 46 liner.

Motorman, I took a look at this awhile ago.  If you've got the equipment it is fairly straight forward.  The toughest part would be squaring up the block on a lathe and boring it smoothly and perpendicular to the crank.

Take another look at the bolt pattern.  I didn't measure it, but it looks the the 40 bolt spacing will require relieving the cylinder flange to fit the bolts.  Or just making an entirely new head to fit with repositioned bolt holes.

It'd be a lot easier to find a beat up 46RC and just replace the piston and liner.