Mower was running fine, started making a noise inside engine, lost power, noise returned, and engine died. No longer will start, just spins. Is the engine trashed or possibly fixable?

Everything is fixable with enough time and money. Practically, as long as the block isn't cracked or otherwise damaged, the internals can be replaced. And, at least it spins and didn't seize. Whether it is cost effective to fix depends on what failed.

You could have broken a piston rod. You could have dropped a valve, you could have sheared a flywheel key. Many possibilities here. You will have to do some investigation to see what is going on.
Remove spark plug and see if the piston moves as you turn the engine by hand. Then do the same with the valve covers off to see if the valves are operating. Then check the flywheel key.