53 hours is not a lot. 9 years is, but even with no oil changes, as long as it had enough oil, it should have been okay.
Is it new to you, or have you had it since new?
What maintenance have you done to it over the years (be honest, no judgement here)?
I would start with the simple things first. Does it have too much oil in it?
Can you hear where the thudding sound is coming from? It could be a hole in the exhaust or muffler. That would probably be easier and cheaper to fix.
I did a leakdown test on my X300 a couple of weeks ago. I didn't open the oil fill valve before I did the test and oil spewed out the breather into the air cleaner. It made a huge mess. I was unable to clear out all the oil from the carburetor, so it smoked like a serious fog machine for at least 10 min before it got most of it cleared out. The leakdown test indicated the engine was fine (almost new at about 2% leakage). I can't explain why that much oil came out on the test. Maybe the breather filter/chamber needs cleaning. Maybe the constant high pressure was too much for the breather system to handle. I just wanted to know that my tester was working right. Now I am still not 100% sure, since all my engines test like new, and some have 300-400 hours on them. But, the tester is working as far as I can tell.
If you know a good honest small engine mechanic that you trust, they can give you an idea of what might be wrong.
The leakdown test will tell you where stuff is leaking. You can hear the leak. You will have to take off the valve covers to get a good idea. Be prepared for dealing with stuck gaskets that can be a pain to remove. Removing them can damage stuff, but many people are capable of dealing with this issue. It just takes time. Depending on the availability of a mechanic, it could take more or less time to go that path. It will probably require more dollars, but sometimes it is something simple and a trained professional can fix it faster and cheaper than someone that is "learning on the job".