Part number you need is M4187 but is obsolete now from Mountfield. Only way you will get an new one is if a dealer has one already in stock.
In term of age you are looking at around 1994 at the very latest as after this date all mowers had the engine safety cut out fitted which yours doesn't have. So at least 16 years old.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix lawn mowers.