Well in that case if it were mine I'd grit my teeth and pay the £300 for another engine but try and get one from a brakers with a reciet and make sure you can return it if it's no good (i bought a secondhand engine for my 250 years ago which was seized so the brakers took it back).
Then once you've done that decide on weather or not you want to keep the bike.
Always do bikes up before selling cos it makes a lot of difference when done right. If you want any tips give me a shout
