Hi Mick, welcome to Outdoorking.

Please post a couple of pictures of your mower, plus a couple of closeups of the handlebar control for the clutch. I doubt that many of us are familiar with Morrison mowers, so I think this may have to be worked out from first principles.

Your Briggs & Stratton engine is 8 cubic inch displacement (135 cc), of the first design generation, with a Pulsa-Jet carburetor, plain main bearings, and a rewind starter. It was made on 19 July, 1989, so if it is the original engine, your mower was probably made at the end of 1989 or early in 1990.
Here is the engine's illustrated parts list:
http://bsintek.basco.com/BriggsDocumentDisplay/Z6ntABVJ1DajI.pdf

Here is its operator's manual:
http://bsintek.basco.com/BriggsDocumentDisplay/flivAJX-nfBhU7y.pdf

The Outdoorking manuals area has an illustrated parts list for the 2008 version of the 400, 500 and 660 reel mowers. I don't know if it would be similar to yours, but it looks like this:
[Linked Image]