New engine for Howard Yeoman rotavator

...oh dear... I'm illiterat !

Is that a side-valve 359cc one cylinder 4-stroke low (6-7:1) compression ratio engine?
http://www.ebay.co.uk/itm/Villiers-User-Manual-Mk20-Mk25-Mk40-/170675556866

To give a fair suggestion one will need to know some thing more about the Villiers engine:
- hp
- torque
- operational revs

I stumbled opon this company in Denmark:
http://www.texas.dk/
They seems to have good experience with engine changes on old machines.

This is a corresponding company in Sweden:
http://www.goransons.com/?page=products/motorer

Point:
 - why choose a petrol engine when you can get a diesel?


My personal experience with engine changes:
(Snowblowers that I have had to change engines on.)
- Tecumse 6hp side-valve is far less powerfull than B&S 5.5 OHV engines.
- B&S 5hp side-valve engiens are -weak- compared to B&S 5.5 OHV engines.


http://www.wealdpower.co.uk/Ref%20Guide/Manufacturers/Villiers_P1.html
http://www.asecc.com/data/villersdata.html
http://www.users.globalnet.co.uk/~pattle/nacc/arc0596.htm
http://www.localhistory.scit.wlv.ac.uk/Museum/Engineering/Villiers/Villiers11.htm

http://www.villiersparts.co.uk/
http://www.villiersservices.co.uk/

I simply can not find any data on the MK40 engine on the net - hence I have poor conception of the engine. Looks like a 5hp low torque max 3000rpm engine  - am I guessing correctly?  If I am - a 6.5hp Chinese in-expensive (about 100-150 pound sterling) will work nicely as replacement engine.
E.g. http://www.jula.se/motor-725017