You can use the O rings that come in the annual maintenance kit, the diameter never changed between the first generation stoves and the latest generations.

What has changed though is the fuel line on the Whisperlite is now a braided metal which is more durable, less prone to leakage. The pump has also changed significantly and the older pumps have a tendency to break where the plunger goes in the pump body and where the check valve at the other end of the pump goes into the pump body.

You can't change the fuel line to the new style but you can change the entire pump to a new one, for a price.

Reality is get an new o ring and use the stove cautiously and hopefully get some value out of it. I have a first generation Whisperlite and it is only kept for collection sake and I won't use it because of the hazard of the fabric covered fuel line and the easy to beak pump retainers. Hope you get to use yours safely.