A current engine mounted upright wouldn't work imho.The power characteristics are wrong.
Most engines are short stroke and produce big (up to 75 bhp) power high up the rev range(maybe 11,000 rpm..)
When they hit dirt they take off,upright or not.
I'm no expert but you can soften off the motor but its still a short stroke revving it's nads off.
It's more complicated than sitting your engine upright.
Long stroke,60bhp and lower revving might help on deep/tricky tracks,but no riders gonna try it.
Cost a lot with no guarrentee