hi mike, really sounds like a bearing has let go, i think you are correct as it sounds like a big end bearing, either way you have to pull the motor and to be honest for the cost of a jap used motor which are generally pretty low milage and fitting i think its a lot better option than messing around with a conrod bearing, the bearing sounds bad enough that it has probably damaged the crankshaft so it would need to have the crankshaft pulled out and then regrind the journal,this would cost you a fair bit so i reckon all up the used jap motor would be a better option, reliability wise and cost w