Ah OK. So that is what we call the Mk1 engine, and it was introduced in 1949, lasting until December 1952 - so it's earlier than the engine that would have originally have been fitted to the bike you are looking at.
As to value, people are asking £10-15,000 for a Mk1 in good running condition, but what they sell for is anybody's guess. As a guide, I bought mine about 7 years ago, in bits, for £3000; I've spent another £7000 or so getting it up and running, and you MUST take the engine down and check the sludge traps before starting it up, even for a short time unless you know the machine has been in regular use - so that has to go into the budget. The value of the machine you are looking at very much depends on its condition and the presence of a number of key parts, such as mudguards, wheels & tanks.
Can you post a picture?