How easy (or difficult!) would it be to obtain the original parts, particularly the hubs?
Because Cleland Aventuras were mostly made from 'off the shelf' parts finding correct parts is not impossible. The hubs came from French Mopeds and getting similar hubs is easy though getting freewheel compatible hubs with the correct fittings for the frame braze ons may be very tricky indeed. I do not currently know of a source for these hubs. Though finding modern 650b rims will be easy it may be very difficult to find the original Super Champion rims or even other age related 650b alloy rims.
You would also need:
*A first generation Shimano DeoreXT (Stag Head) groupset and brake levers.
*replacement period: seat post, stem, mudguards, headset and grips.
*Modified TA long triple chainset with teeth removed from the outer ring to form a guard.
*To fabricate the missing bash plate and chain guard. (I can provide patterns).
*And maybe original Nokia Hakkapelliita 650b tyres that have not been manufactured since 1993.Below is an image showing an original spec Cleland Avenura
Apart that is for the Brooks saddle. Because Clelands were originally supplied with cheap Iscaselle sprung saddles.
File comment: Original Spec Cleland Aventura
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9/16" Suntour BMX MP-1000 pedals / 56 tooth TA 6-bolt chainring.(I'm not bothered if the teeth are worn or damaged)