you can also convert them to horisontal shaft engines. most vertical shaft push mowers ive seen have either stepped shafts or 7/8" keyed shafts. the best would be a 3/4" shaft with a 3/16" keyway, but you CAN use 7/8" keyed and 1" keyed.
so to convert them to run sideways you need to do the following:
make a vertical engine mount
make a oil slinger on the connecting rod
make a custom intake manifold to mount the carb upright
rotate the pull start
im going to do this to a 6.75 OHV briggs as soon as i finish my kart. it will probably just be used to say that i can do it... wont have much of a purpose because i doubt it will be very reliable.