Yep, you'd need the whole front end.
If you were trying to put a Wee front end on a Vee for some reason, you wouldn't need the speedo sensor -- the Vee's speedo sensor is on the front sprocket. You'd have to leave the sensor in place, but it would only function as a spacer so you'd just clip the wire or machine a substitute.
Suzuki presumably had good reasons for using a larger axle -- the Vee is heavier and a lot more powerful. But I guess you probably wouldn't die right away or anything. Lots of earlier Suzukis were even heavier and used even smaller forks, axles, and bearings.
If you tried to put a Vee front end on a Wee, then you'd have the problem of how to make the speedo work. If the reason(s) for making the swap involved destruction of the rest of the front end, including the instruments, then you might not care if you're using something else, like an aftermarket computer or a bicycle speedo.
There are about a zillion Wees for every Vee, so I don't see the latter scenario happening very often.
2002 Suzuki DL1000 V-Strom, Dark Metallic Space Blue
1983 Suzuki GS850G, Cosmic Blue
2005 KLR685, Aztec Red - Turd II.2, the ReReTurdening