A stud should be a standard part, like a bolt. All my yanmar stuff is in storage, but I know the YSM 12 manuals are on line. The stud should have a Yanmar part number and a description like m6-16 x 65. So the M is metric, 6 mm in diameter, 16 threads per mm, 65 mm long. You can pick up a metric tap and die set at Harbor Freight or such, and with trial and error you can figure out what size you need. McMaster Carr proably has the stud you need, if not the local auto parts store. If you can get the stud out it shouldn't be hard to match up.
L