the rod doesnt do anything for the stroke.
the length of the rod changes the following things:
- weight (lighter, hence shorter gives more power)
- forces on the cilinderwalls (leibaankrachten in dutch; the longer the better, because the angle of the rod is smaller)
- height of the cilinders, because a longer rod will mean the piston is located higher
if you want the rod to influence the stroke.... get a rod which is made from aluminium. it will be softer and because of that it will strech more when at the top and it will shorten more when at the bottom, giving a bit more stroke
and it wont be able to withstand the forces very long so it will break very soon...