Get that leak fixed, then worry about what's up with the power. If you're low on hydro oil, that could feel like power loss. Check around the rear axels and see if it's coming from there, or check all the hydro lines. Rubber tends to rot after 30 years.
Service manuals can be bought from JD, or sometimes ebay.
Get familiar with the JD parts catalog...
http://jdpc.deere.com/jdpc/servlet/com.deere.u90490.partscatalog.view.servlets.HomePageServlet_Alt
enter your model # and hit find. Look in the sectional index.
Welcome to the forum.
Service manuals can be bought from JD, or sometimes ebay.
Get familiar with the JD parts catalog...
http://jdpc.deere.com/jdpc/servlet/com.deere.u90490.partscatalog.view.servlets.HomePageServlet_Alt
enter your model # and hit find. Look in the sectional index.
Welcome to the forum.