I have manuals for machine and engine, drop me a pm with your e mail address and I will send copies. From memory the only clutch is for drive to wheels. Cutter bar may be tight making it difficult to turn engine, well worth removing belt to see and free up cutter bar if necessary.
Are there any patent numbers on the engine? The original Rotoscythe was made by Power Specialities of Maidenhead. Looking on the patents website might give a clue or answer.
Graham welcome to the forum. The Museum of English Rural Life (MERL) at Reading have records for Allen Scythe machines and should be able to give the information you require.
Your question would have been better as a new topic, avoids two similar but different topics getting mixed up.
That is a very useful link, loads of information.
There is a Clinton Service and Maintenance Manual (revised 1978) in the VHGMC archive which contains service data and parts lists for 400 series engines. It is about 160 pages in total.
I have a manual for MK40 (not HS version) which gives points gap of 0.012″ – 0.015″ and timing is 1/8″ before TDC; this is best measured with cylinder head off. Timing is set at factory with shims under under breaker body.
If breaker body has not been disturbed and points gap is correct then timing should be OK.
That looks like a Howard Gem. A quick google search brought up this copy of the service manual which details everything about stripping, repairing and rebuilding the Gem.
I am sure some parts will be of use as spares. Listing it via an online auction is one option and will have a wide audience.
That is an interesting find. Looks as if it should clamp onto a range of wheelbarrows, would be good to see a photo of it fitted in working position. Please keep us posted.
Welcome, Briggs & Stratton parts are available from various suppliers a good place to try is Briggsbits . Note they will only have engine parts, your rotavator was not made by Briggs & Stratton . A photo will help identify what your rotavator is.