Ryobi s430 oil leak

Well my experience with these 4 cycle is not good as they are of a very poor oiling design. It is mostly likely piston blow-by caused by worn piston, rings, and cylinder. Personally it is not worth putting in $145 plus other parts for an obsolete engine.
 
The four stroke trimmers that I have seen that work as intended are the hybrids. There is Matika that seems to work as a true 4 cycle but they use a pressurize oiling system which is different than the Ryobi and Troybilt oil slinger system.
 
Sorry it is just my experience from working on these on a regular basis.

Yes these were only design for level operation but operators including myself can and will use a handheld at odd angles in order to get  some of trimming work done. Personally I have very steep embankments that must be trimmed out or the state and county will try to do it gouging deep cuts into it causing high erosion.
 
As for operation I have seen even the handheld 4C bowers to lose an engine due to oiling problems. These I know are used on near level operation as you just blowing things around and no need to operate on it side or upside down.
 
As for the seals since the OEM don't provide them it will hard to get them and replacing them even they were available it will be a pain as you got to disassemble the half crank to gain access to the inner seal as the ones that I have taken a part have two seals. This requires a hydraulic press to do.

MdStamen
I got rid of mine. Not worth $199. Also It is not a full crank. If you want a inexpensive 4 stroke The Troy bilt is a full crank and has the ability to start with a adapter and a drill.

Even they are pieces of junk. I have seen so many that when one comes to my shop I just turn them away.