My envelope math is that engine plus change labor is 50K and it is rated for 2000 hours. That’s 25 per hour so value drops 2500 for 100 hours.
You could also do a 15 year straight line depreciation on the whole value starting at new cost and ending at some much lower price at the end.
Straight line will make more sense to the assessor but do what they will in your own self interest.
Also can just keep the assessment at what you insure the hull for. Should be a good metric but you might insure for replacement cost instead of actual value.