Yes I remember the L51C and DP51 quite well having changed a few clutch plates on them in my time. You don't need to take the engine out or transmission to do this repair, remove the top of the clutch and fluid fly wheel housing, I can't remember whether or not you remove the cross shaft or not, lossesen of the clutch diaphram and remove the clutch release bearing return spring witch I always found to be a mystery how to refit, move the clutch diaphram back and you will see a circlip located on a shaft that runs through the diaphram and clutch plate and locates in the fluid coupling,release the circling, at the end of this shaft is a threaded hole, find a bolt you can screw into it and weld a pull ring onto the end of it, screw it into the shaft, pass a bar through it and you will be able to pull the shaft out enabling you to remove the diaphram and clutch plate, reassemble in the reverse order, before you do any of this check the fluid coupling level a telus hydraulic oil is all you need to top it up if low. I think it's telus 21 oil.
Happy days ?

Regards Titus