We could hear the starter motor turn on, but it never engaged the flywheel to actually start the engine. So, we're thinking that some part of the starter is probably toast.
Can you hear the starter spinning, or does the starter starting make a loud click, and hum. I am trying to verify that the starter, and not the engine, is the problem. Make sure that you can turn the crankshaft by using the compression release, and pulling on the V belt.
Denise and Chuckles are right, the the marina will charge $80+ per hour (and the guy doing the work will likely get ~$15/ hr) to do the job.
A good place to look for a starter is your local Kubota tractor dealer. A Kubota part number for my M25 engine (which is related to the M5416 engine*) is; 15231-63015. This number is probably obsolete, and may have "-" in the wrong place, but the Kubota parts guys can get you the current part number.
Unless you want to pay 4x the tractor prices, get the cross reference guide for YOUR motor.
* From the History of the Universal Motor Company
, which is sprinkled throughout the internet;
The first Universal diesel was advertised in 1976. Designated the Model 5416, it was a 45 cu in, 16 hp, 2 cylinder, based on the Kubota Z-751 block and built from 1976 to 1982. Approximately 1,000 were sold. It was replaced by the Model M20, which was produced in 1982 and 1983. Approximately 230 were sold.
The Model 5421, a 52 cu in, 21 hp, 3 cylinder, based on the Kubota D-850 block, was built from 1978 to 1981. Approximately 1,000 were sold. It was replaced by the Model M25, which was produced from 1981 to 1986. Approximately 3,650 were sold.