I only put a new rubber gasket twice there in the last 20 odd years, handle it with care and when not damaged you can use it again. Afaik they're still obtainable from Honda, or else look on
www.cmsnl.com for the number, then search by that number on Google for the nearest to your home possibility...
Chain slack is set on the mainstand, the front sprocket sits quite some distance from the swingarm pivot point so never set it too tight, you can always check it after setting by sitting on the bike , especially if you're one of the weight challenged kind