Origional 90 transfer box ratio should be a 1.4,discovery a 1.2,fitting the 200 tdi and 1.2 ratio into the 90 should not cause a problem unless you have big tyres. If the 90 has big tyres or tows a heavy trailer a 1.7 ratio box helps. To check what ratio is already fitted,engage 4th gear,engage difflock in high box,jack up front and back wheels on one side,release handbrake,turn engine with a socket to take up slack in transmission,mark crank pulley and handbrake drum,rotating engine now 1.4 turns should give one full turn of brake drum,other way is to look at sticky label next to filler plug and see what it says!