Introduction - British Anzani

Just to introduce myself and my newly acquired Lawnrider. Much to learn ! Can anybody tell me roughly what year and model please ? No aluminium tag is visible, twin drive wheels and 2 levers on the handlebars. The plan is to restore, run and use. Lose the rust and keep as much green as possible. Two questions - amongst many - the engine is a Villiers 515H (I think) but I suspect it's not the original. Serial number begins 760E. Would this/should this originally have had the Briggs and Stratton engine and if so what model ? Would it be a straight swap ? (Check my name for the reason for the questions :-)  )