Montesa tanks up to about 1976/7 were fibreglass rather than plastic. I have a 60's kit car to rebuild which is a fibreglass monocoque with the tanks moulded into the monocoque. Other owners have experienced problems with unleaded fuels attacking the fibreglass which has caused blocked fuel lines where the fibreglass has turned to a kind of jelly. There are also known issues with unleaded attacking some of the seals in the carburettor on older classic cars which is why I felt inclined to ask questions rather than plough ahead. Thanks for all the advice. I think I will get the bike tidied up a bit and then see if I can get her started. The bike is still very original so, having looked at the price of original parts, I will change things like the mudguards, if I decide to compete again, and keep the originals to put back if/when I decide to sell.