Congratulations on a workhorse of a machine. I just got done doing a HF tune up on our 1336. It is now a 1636. Also did the homemade impeller mod, I used a semi truck mud flap & 5 stainless steel bolts & hardware on each impeller blade. There is no space now between impeller blades and housing.
I used the Harbor Freight 459 cc engine. No regrets. Feels way more stout than the Tecumseh 13 hp it replaced. They are fantastic machines.
Mine always steered very hard, right from the factory. I took the automatic differential apart. It looked way too complicated for me so I filled up the housing with gear oil, hoping that the gear oil would soften up the old dry-ish grease that was in there. It is 100% easier to steer now. Seems way better than new. I am sure that it was not designed for gear oil, but nothing has leaked out yet. There were so many little parts inside, I just thought that was the simplest way to go about it, without getting in over my head. Should have done it years ago.