lost the 'steerer'...? Do you mean the handlebar stem?
Mostly (in UK/US/Japanese built bikes) they are 22.2mm (7/8") dia where they go into the fork. However French ones which also use a 25mm (instead of 1") headset thread tend to use a 22.0mm dia stem instead. These are not that easy to find. Nor is it the only possible alternative size, either.
So do try a normal 22.2mm stem but if that doesn't go in, it might well be that you need a 22.0mm one instead.
cheers