these motors are very easy to pull apart and rebuild. I see your 300 is built with the same architecture as my smaller 163cc. I found it easiest to remove engine still connected to rear wheel and roll the whole assembly out. Take detailed notes on what order you disassemble everything, take pictures and bag and label all the parts that go together.
The gaskets and seals are relatively inexpensive. Might be a good time to replace cam chain guides, too.
The absolute biggest pain in the whole process was scraping the old gasket material from the aluminum cases. There's no secret sauce, just time and sharp razor blades.
Reassembly is easy but I'd recommend having a shop manual for torque specs, etc.
