The guys who build board track bikes have the barrel vertical. Dip stick hole needs to be well sealed, and measured quantity of oil rather than using  factory dipstick.
Does that help Graham? (There is a vertical version of some of the Hondas, but I have yet to see a clone one.)
Nice pun BTW Mr Hill, I got this one for a change

Don't try to tell me that "inclined" was inadvertant?

I have no idea, how ever quite interested...
A in the answer, might save me some room.
B quite interested in Why Mr Hill is asking
What you got planned???
The only info useless or otherwise re lubrication of these engines was
keep the oil fresh and so long as they get to bounce around a lot
the engine will get lubed?
So maybe something for those with rear suspensions to think about!
 

Depending on the details of each clone most have a well that the oil get splashed about from, and other areas like the crank bearings catch the splashed oil and feed it through a little hole.   I guess changing the mounting angle just needs to allow those mechanisms to still work.

Most seem to be using the Mikuni as far as I know, and that could be pretty achievable. I second what Bill has questioned.

As an aside briggs,kohler,tecumseh etc all made a decent run of 7hp vertical side valve engines which imo are very much of the 1930s era lol

I’ve only found vertical crank side valve new engines - do you know if any make horizontal crankshaft side valves?