Can I fit a 14inch bar on my MS 170 chainsaw and if so will it be detrimental to the performance and running of the saw.

I have had a look and we do have a 14" bar listed for your machine: https://www.lsengineers.co.uk/catalog/product/view/id/10112 and here is the chain you would need to go with it: https://www.lsengineers.co.uk/catalog/product/view/id/32556