This is really the true answer. You're going to get an occasional year where you get a large maintenance bill, then you'll be fine for a few years, then you'll get a big bill again. Just average it out over time.
A couple ways to make it feel better - they say the annual expenditure of your boat will be 10% of the brand-new-boat purchase price. Not the used boat price. Assume your boat was $250,000 new so budget $25,000 per year. This should cover insurance, slip, maintenance, fuel for a 100 hour per season normal user. If you're paying less than 10% of new boat purchase price you're winning.
Another way to make it more bearable is look at it this way;
Assume 100 hours cruising diesel fuel consumption at 12 US GPH for diesel. That's 45L/hr, or 4500L per season. Diesel is $1.50 per L for $6750 per season.
Assume 100 hours cruising gasoline fuel consumption at 24 US GPH for gasoline. That's 90L/hr or 9000L per season. Mid grade marine gasoline is $1.75 per L for $15,750.00 per season.
With the diesel engines you are saving $9000.00 in fuel per 100 hour cruising season.
Now take a look at a repower project - assume your repower is $100,000 net after selling your existing drives and motors. Assume you can make 10% with that money in a simple investment like an index fund or ETF that you don't need to manage. That $100,000 that you put into your motors is a $10,000 per year opportunity cost.
By sticking with existing motors and drives you are saving $19000.00 per 100 hour year, and you don't have a $100,000 capital expenditure that you have to pay back.
Keep existing, do some more preventative maintenance. Save $19,000.00 per year less the occasional increased maintenance. Keep the amazing range the diesel engines give you, the hot water you get, etc and when it comes time to sell the boat you won't be priced to recover the $100,000 you spent on the repower.
