I replaced some o-rings on my new to me second TW because of seepage. Here are the part numbers and how they fit (IMHO).
Cam sprocket cover o-ring: 2-155 N-70
O-ring is a little short but will stretch to fit and should work well. .36 cents each.
Drain plug o-ring : 2-221 N-70
Excellent fit. 19 cents each.
Large oil filter cover o-ring: 2-227 N-70
O-ring is just a teeny tiny bit to big but can be used. It should work well. 21 cents each.
Small oil filter cover o-ring: 2-108 N-70
Replacement is a teeny tiny bit to small but should work well. 6 cents each.
Valve cover o-ring: 2-140 N-70
Excellent fit. 21 cents each. Remember to order at least two of them.
Small blind plug o-ring: 2-113 N-70
Excellent fit. 18 cents each.
Big blind plug o-ring: 2-124 N-70
Excellent fit. 30 cents each.
Plastic cam chain adjusting cover: 2-115 N-70
O-ring is a little large but can be pushed into the groove and stays there. 18 cents each
Plastic oil fill plug: 2-116 N-70
O-ring is a little small but stays on plug when unscrewed. 2-117 is larger but will fall off. 18 cents each.
Starter housing o-ring that seal starter to engine case: 2-120
Excellent fit ! Should last a very long time and only costs .11 cents each.
Carburetor float bowl gasket: 2-037
2.5" in diameter and 1/16" cross section. Seems to fit well enough but you have to use High Tack gasket sealer to hold the o-ring in place. $.70 each for Buna (Nitrile) and $2 each for Vitron (better in gasoline)*
*(1-9-'14) I think Ace Seal has wised up and is charging more to people that just buy a few o-rings at a time. Grainger is cheaper per o-ring but you have to buy in quantity.
I bought them at Ace Seal. www.aceseal.com They do mail order via email or phone. $15 minimum order for mail order -- no minimum if you walk in. I don't know about S&H.
I spoke with a guy that works there and he said if I wanted to upgrade o-ring material (higher temperatures) then to go with the Flourosilicone (L) or the Viton (V). The cost is 3 to 4 times greater than the Nitrile (N).
Long live the internal combustion engine!