Hi Ewan, other options which I would follow is, if you have a parts manual for the engine, do a search online using the part number.
Often, something will show up with an alternative bearing number or dimension (probably stateside) to work to.
Or, post the question on Redsquare, as there must be owners of early 60's machines who have been there and done it .
They should be same size bearing as in K181.
Good luck
.