I can't advise you one way or the other as tastes are so individual, but basically, I'd probably buy it for that if it was offered to me. ;o))
It is a fairly uncommon engine and was made (according to the "A-Z") for thirty odd years from the thirties to the sixties. Although they never made the Coburn, Petters took over spares supply for ten years, so the spares situation might not be entirely desperate. That said, if I was buying it, I'd not bank upon being able to get any spares other than by making them or causing them to be made.
A crucial matter is the magneto. If it is missing or cannot be induced to produce a regular spark, then a replacement is going to cost you at least as much as the engine. £40-£60 is not an unusual price to pay for a decent magneto these days.
In all likelihood you are not buying it with the intention of selling it on, but bear in mind that it is unlikely to fetch more than perhaps £100 in restored show order as air cooled engines are not as popular as water cooled - why, the good Lord alone knoweth!
Don't hesitate to ask further questions should you need to do so. This NG needs something to get it's corporate teeth into!