you have a through hub exhaust which means thats where the exhaust exits,the water should come out of a tell tale on the lower cowling or maybe from a port part way down the back of the lower leg.50:1 oil mixture and any tcw-3 rated 2 stoke oil will do by rights.hopefully somebody else chimes in about the water exit as im not 100 percent positive which one i described is correct for your motor.i think if you put your hand just below the bottom of the cowling where there is a couple of ports you should feel water mist ,i was looking at some vids on youtube and i only seen one 1975 with what looks like a home made telltale.
if it only can out at the prop how would you tell if its working in the water?hence the need for a telltale or the misting from the ports i mentioned.i have at least 6 or 7 motors and they either have a telltale or port exhaust, it might be that its not very noticable on muffs thats all .if your unsure of the history of the impeller your correct in replacing,cheaper than a motor.dont do what some of those characters did in the vids and rev the motor when its not in the water or you could risk a runaway and total boom boom.you might get away with it a bunch of times but that one runaway is going to be messy.
