Hello and welcome to the forum.
As far as I know a plough was not available for the Howard Gem.
Looking at your photographs I can see it has been modified with a battery and ignition coil, presumably because the magneto has failed. Parts for these are available in the UK.
I have problems with the Albion gearbox on my Trusty Earthquake. On initial start up after a period of storage the clutch sticks and drags.After a short period of running it operates normally. I have always assumed it is the corks in the clutch plate sticking. No kick start on gearbox.
Do you know which model of Albion gearbox it is? I do have some manuals and could send you a scanned copy if I have one for the type you have.
Machine 624 was despatched on 12-7-55, the agent was Farmfitters, unfortunately the customer details are not recorded for this machine. Series 3 production ran from 1954 to 1965.
I have manuals and engine information, Geo Monro is my speciality. I also have the factory records and can date machines from the machine number. pm sent with more info.
Green Spark Plug Company will list alternative spark plugs. They also sell HT cable and terminals.
The Museum of English Rural Life (MERL) at Reading hold the factory records which will give you date of machine.