Hi Dom,
If he gets a recent copy of 'Old Bike Mart' there are lots of sources for early Brit stuff and if it's engine parts, there's a Villiers specialist who usually advertises in there. Cycle parts and tin-ware might be more of a problem though. There's also a guy in Nederweert in the Netherlands (Yesterdays.nl) who turns up lots of early stuff - including Hondas. Mostly complete bikes but might know of spares somewhere.
Ian