The legality of the changes that you have made to this valve basically depend on what the changes are, and what part of the world you are in. Here in Europe all valves supplied must meet the requirements of the Pressure Equipment Directive. If you have changed the pressure boundary in any way...
Sand and PTFE do not go together and should be avoided if possible. If the valve type is a globe (rising stem) then it is possible to protect the PTFE by building it into the plug and flowing over the plug head. In this way the PTFE does not see the highest pressure drop and the area of high...
Try going to a company that makes both types of valve. Weir Valves and Controls manufacture both gate and globe valves. The web site is:
http://www.weirvalve.com/