Re: 35 Johnson gear case
Depending on how badly and where it is cracked I would be very tempted to just JB Weld the old case, I had a huge crack in an old 9.9 I had and the JB held fine for many years.
A lower is a transmission and has very tight tolerances and requires special tool to do correctly so if you can save the old one I would reccomend that.
If you do decide to go it alone you absolutely have to buy a manual, factory OMC not Clymers or any other one, you can get them here at iboats or on Ebay