The timing gear and roller bearing won't go out through the oil seal hole in the block. You have to pull the crank out of the engine. I think from the rear, if its an R.
I am thinking you will have to get the inner race of the roller bearing off in order to change the crank timing gear.
I put the crank gear from an RE (ZT) engine on the crank in one of my Rs with no problems. Probably the crank gear from any RE, EE, or L-head 206 will work. (The crank gears from the 206 and 220 OHV engines may work too, but I don't know.)