hydrolic fluid

These tractors will piss out some hydraulic fluid when going up hills when the reservoir is full.  Inherent in the design.  You can modify the fill plug to hook up a longer breather tube to go up the ROPS arm a ways and beat the problem, several guys have done that.

Nope, you don't want to replace that flow control valve with a plug.  It is necessary that it be adjustable to control the fall rate of different weights of implements.  There is a small Allen-head set screw on the side that will lock it in place.  Might have to scrape some paint to find it.  Set it correctly for the weight of your implement and then lock the set screw.  If you just use a plug, it will be wide open all the time and your implements may come crashing down and  be damaged.  Bad idea generally, and really bad with ground-engaging implements that might then damage the lift mechanism itself.