Oh dear! Someone in the past has made a mess of that eccentric strap and its studs!

It looks as they have bent those and strained the strap by forcing the nuts down onto curved surfaces that really needs little flats for the nuts to bear onto.

If the rod is not distorted, and assuming the division is perpendicular to the road axis, as they usually are, it’s possible to measure the centre distance quite easily.

Remove the rod from the engine. Stand it vertically by its flats on a surface-plate (or similar, such as milling-machine table) supported by an angle-plate, measure the height to the tangent top of the pin, then subtract the pin radius.

Alternatively, use a vernier to measure from the pin to the crown of the internal arc, subtract the pin radius, add the sheave radius.

If I had an engine with a broken or missing strap I would go about finding it’s length as follows.

Remove the valve chest cover so you can see the valve etc.

Rotate the valve rod to make sure there are several turns of thread available each side of the rectangular nut.

Position the valve so it is central to the ports

No measure the distance from ctr of the crank shaft to ctr of the hole in the valve rod clevis. This will be the size to make the strap/rod ctr distance.