Troubleshooting help needed re: sudden engine stoppage

[JetBoat1A]

New to the forum and thanks in advance to those of you who chose to try to help answer my question.

Background & setting:
I have a 2001 Challenger, dual marine 718cc marine engines new to me this season. Yesterday, while cruising on plane with both engines apparently running normally, the boat suddenly made an uncommanded turn towards the right and I immediately noticed that the starboard side engine RPM gauge had dropped to zero and that that engine was no longer running. When this happened both engines had been running at about 4200 RPM and the boat was skimming along gloriously. Then suddenly nothing from the right engine. Attempts to restart that engine were unsuccessful and I limped back on one engine.

Initial observations:
With battery master on, fuel switch on, 2 beeps when the "key" is placed .... left engine starts and runs normally; right engine when I press the starter button there is a
clear clunk sound followed by nothing, the engine does not turn over at all.

Question:
How do I begin to sequentially trouble shoot this?
My first concern is finding out whether this is a mechanical or electrical problem. How can I tell if the engine is seized or if it is just an electrical or starter problem?
I am fairly mechanically inclined in regard to autos but know little about jet boats or marine engines. So I appreciate guidance from those of you experienced in these jet boats and Rotax marine engines.

Thanks in advance for your helpful responses.

 

[2x2smoke]

Inspect the jet pump for a foreign body. Inspect the bilge to see if the crankcase is breached. See if you can turn the PTO with your hand to see if something is locked up. If so, pull off the jet pump to see if its the pump or the motor that is locked up. Pull the spark plugs looking for metal flakes or other signs of engine destruction. Pull the cylinder head off to see if the top end exploded. Pull the magneto cover to see if the flywheel exploded.

If nothing is locked up then you will skip most of the invasive suggestions above and start to look into electrical causes but i can't really think of an issue that would stall one engine and at the same time mess up the starter

 

[JetBoat1A]

Unable to turn the PTO by hand on the nonworking engine, can easily turn it on the good engine for comparison.
Removed the plastic PTO shield and see rubber boot & grease zirk. How would I "pull off the jet pump" to further check whether it is the engine or the jet pump that is locked up.
Looking with a flash light from the rear of the jet nozzle I do not see anything stuck in the impeller. There is a chip out of one of the impeller blades but not sure if that was there before the engine stoppage.

I am very bummed out.