Hello "Pan" Lewandowski,
Sorry about the troubles you are having but, let us see if we can fix it up for you.
First, after thinking about it for a while it, Make sure you put the correct amount of oil in the transmission. Your transmission takes a maximum of 2.5 Quarts NO More than that. If it is over filled, the oil will flow out the rear seal. Please make sure of this before you start taking anything apart. If the correct amount of oil is in the transmission please try this:
1) unbolt the propeller shaft from the transmission and slide propellershaft back as far as possible (8'' to 12" is fine) BUT watch the shaft packing gland excessive leaking. You might have to tighten packing gland a little, not too much.
2) Take a 1 1/2" socket and remove the the Flange Nut on the back of the transmission. You will have to find two (2) old bolts to put in the flange bolt holes and a bar in between the bolts to hold the flange from turning when you take the nut off.
3) After the nut is off, pull the flange off the transmission. It should slide right off.
4) Find a Socket the same size of the seal (see picture) put the socket against the seal and TAP it in about 1/8" inch NO more. if you do not have a socket use a small punch.
5) After this is done, slide the flange back on, install the nut (tight) , pull propellr shaft back in place and bolt it back together. Recheck your packing gland for leaking.
I hope this helps you my friend. If it does not work call me and I will have another way to fix it but it will take more time to repair.
Call me any time
Your Friend Marty