Your local shop will know the size. I believe it's a .458" . Get a screw in stem. That way you don't have to put the plastic support back on. Plus: it'll last for many tire changes. Why do you need to change the stem without taking the wheel/tire off the bike?
It's a friend of mine's valk, the rubber part of the stem is cracked and leaking, the tire does not need replacing. I thought if it can easily be done w/o taking the wheel off, that would be the way to go.
As far as the stem, I bought one from Jake Wilson for myself, along with a new tire. It is the 90 degree chrome screw type. I have not measured it, but assume that they all should be the same diameter. I know lots of folks here have ordered from Patchboy, but as long as I was ordering from Jake Wilson....