I would be worrying more about getting new rear engine mounts than changing a serviceable air cleaner that, using your words, was fitted by Mr Morris (Except it was not, it was fitted by Newage/Tempest)
The video clearly shows the front mounts and they are the standard Newage/Tempest system that employs the standard vehicle rubber mounts and those rarely fail.
The rear mounts are obscured but from the part of the flywheel housing that I can see it looks as if the flywheel housing, of which the engine feet are a part, is also the original Newage/Tempest one. Now the mounts used in these, that look like a tall top hats passing up through a large hole in the foot with the securing screw/bolt down the centre, were made by Metalastic Ltd and I believe they have been unobtainable for a fair number of years.
That type of mount are secured into the foot by three small set screws, screwed up from underneath, and when the mount rubber sags they hit the bed and produce vibrations and so on. The screws are not under any significant load, so sometimes it is possible to file the heads of the screws to about half thickness or turn up a thick washer that is the same size as the flange on the bottom of the metal centre to the mount and use that to make up for the sag in the mount, but it is all too easy to mess up the shaft alignment if you make it too thick. Both "solutions" should only be considered as temporary.