The BMW Boxer motor has two cylinders horizontally opposed at 180 degrees.
In Contrast Moto Guzzi also builds horizontally opposed twins, but the angle between them is 90* not 180*. So on the BMW the cylinders are "flat" and on the Guzzi the cylinders appear angled up at 45* (per side, for 90* total).
The OP's picture clearly shows the cylinders are at an angle, not flat, so it's not BMW.
Honda built some 90* transverse mounted V-Twins in the 1980s, but this bike is too old (drum brakes) to be one of them.
It's a Guzzi for sure.
“It’s a Guzzi for sure.”
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See:
https://freerepublic.com/focus/chat/4072498/posts?page=316#316