The Kevlar and polyurethane micro V belt should last for at least 100,000 miles and probably a lot more. You can tighten the belt on a mk1 by moving the alternator on its pivot, literally a ten minute job if you have some decent tools - the pivot bolt is under the alternator and the adjustment screw thread is on top IIRC - do not overtighten the belt as there is no stretch in it at all, and overtightening can cause alternator or waterpump bearing to fail prematurely. The belt itself from Honda is about £50, cheaper ones are available IIRC some more info on belts on forum, use search function and belts, auxiliary, or serpentine as key words.
Check the belt visually ( which is all Honda do ) if it is not visibly cracked or fraying at the edges it is probably OK.
Can't comment on belt dressings, but try tightening it first.
Mk2 Jazz and onwards have an automatic belt tensioner which you have to release the pressure on to remove the belt or work on anything the belt drives.