Whats wrong with the old motor? Thats a good one to modify, I am in the process of trying to find the right parts to modify a 5hp as well.
the engine on there is not a 5hp.its either a 3hp or 3.5hp.
industrial plus engines are usually steel bore single bearing engines.very good to build.
contact turk racing and get a catalog. they have a 03ss cam,billet lifters,springs,and billet rod that would make it go real well.
max rpm is about 7500, excellent torque with a torque convertor.
can use a older carb,stock on gas, get yourself a cast iron 3hp flywheel and drop down bracket for it.
you'd be surprised at how well it'll run.
ONE LAST THING..NOT ALUMINUM FLYWHEEL,A CAST IRON ONE.they will hold up to 7500rpm as long as its a decent one,not all beat up.