seriously if you have checked every thing people have suggested its back to get it out and start striping it starting with the fan housing
Unless it's been seriously overheated and the pistons are trying to seize, it won't be anything inside the engine, so as others have suggested, take it out. This is the one that had the wrong size fan? It could very well be knackered already - it's not so much that the fan is smaller, it's the big gap it leaves between it and the fan housing.
Hi Grub,Ian is a fully trained VW Audi mechanic who also services air cooled stuff.He has a splitty with a mega motor in it plus a few other air cooled cars of his own.A phone call will cost you a few pence.