So far I have successfully converted 4 different model Lawn-Boy 2 cycle vertical shaft engines to work in horizontal position. An easy undertaking if you understand how/why things work the way they do.
Oil/fuel mixture means all parts are lubricated all the time regardless of shaft orientation. I have seen jugglers use running chainsaws as part of their act.
A vertical shaft 4 cycle engine is very different for a couple of reasons.
These engines rely on a reservoir or sump to supply moving parts with oil.
A typical 3.5 hp Tecumseh uses a pump powered off camshaft which is submerged in oil. An equivalent Briggs and Stratton uses an oil slinger partially submerged in oil.
It seems to me that the B&S engine could be more easily converted as long as oil slinger is making contact with oil. Tecumseh might be a different story in that pump may be left high and dry and unable to pickup oil.
See my other posts relative to this one on
http://www.diygokarts.com/vb/showthread.php?t=785.