Hi Geoff, welcome to Outdoorking.
There is an illustrated parts list for the Rover Ranger XC 52144 in the manuals section, and it appears to cover the 52150 and 52155 as well. (The main difference seems to be the engine size.) If you have the same model, the cutter is engaged by pushing a lever to the left of the driver's seat downward, then across to the right. The movement to the right locks the lever in that position. You need to trace the mechanism connected to that lever behind the panel. I think you may find that the lever operates a push rod that moves the whole cutting deck backwards, thus tightening the drive belt. The illustrations do not show the control levers or linkages so this is not entirely clear, but it looks as if that may be the case. When the same lever is moved to the left then allowed to rise, the belt is loosened and the brake is applied to the rotating parts.
If you post pictures of the mechanism behind the engagement lever, and its linkage to the system that supports and cutting deck and controls its movement backward and forward, we can talk about it in more detail. The owner manual describes the cutting deck as "fully floating", and considering that it moves both backward and forward and up and down, that seems almost accurate. The owner manual also says that the cutting disk is driven by a "V belt clutch", which seems to conform to my guess as to how it all works, in the previous paragraph. It describes how to adjust the belt so that the clutch system works correctly.
