On the BS90 / BS100 engines, the oil pump simply sprays the oil into the inlet tract behind the carb, with no direct feed to bearings or cyclinder. It relies on crankcase turbulence to mix and distribute the oil - just like on a mixed fuel "petroil" system. Make sure you do cap off the oil feed holes - otherwise you'll suck air in and weaken the mixture. To be safe, it would probably be a good idea to remove the oil pump and its drive shaft altogether and fit a blanking plate over the hole in teh transmission cover. This will obviously entail some dismantling, but I would be concerned about the oil pump innards still spinning around and not being fed with oil from the tank.
If you do go for a pre-mix fuel, stick to a 20:1 ratio, the same as used on the early BS90s and watch the blue smoke trail appear!
Hope that helps.
Graham