I used someone else's Wisperlite Universal for the duration of a trip and it looks like the only functional difference is a jet that can be attached for burning propane. Can I purchase just that jet and burn propane with my Wisperlite International?
This question honestly could be too localized on these specific products. However, considering the ubiquity of these stoves I find it is probably quite useful information to know if they could have interchangeable parts.
MSR's Whisperlite International & Universal stoves are very similar products, but they have some fairly notable differences. Let's call the Whisperlite International the WLI and the Whisperlite Universal the WLU just to make it a bit easier.
Let's look at the WLI & WLU parts first:
taken from MSR WLI product manual
taken from MSR WLU product manual
As you know, the WLU is the newest iteration of this famous stove that is compatible with not only multiple liquid fuels, but also canister fuels. To achieve this the WLU departs in design from the WLI by having two adapters on the fuel line: one to connect to a canister and one to connect to the traditional MSR pump. This means the WLU's fuel line and generator tube are different parts from the WLI. The WLU has a UC jet that is designed for use with canisters in addition to the liquid fuel UK and UG jets used for both stoves. (The older model WLI uses different jets than the current model WLI.) The WLU also includes a canister stand for more efficient burning in cold weather and with low fuel.
So theoretically a WLI could be converted to a WLU with the following parts:
However, purchasing these parts would take a call to customer service of Cascade Designs, as they are not available through standard channels. Though your local MSR outdoor retailer may be able to order these parts for you. The real issue is going to be whether the cost of these parts is actually much cheaper than buying the WLU.
Either way, it does seem possible to convert at least the current generation WLI into a WLU.