Thanks for the pics, phred45, and thanks for the tool explanation, BobL.
The good news is that you already have a rim-type clutch drum. Unless it is worn out, there is no need to replace it.
If the rim turns out to be 404, then you just swap in a 3/8 x 8 rim, which costs all of $3 at Baileys, or a few bucks more at your local dealer.
$3 rim
If the rim turns out to be 3/8, then you are good to go, except ....... it's a good idea to install a new rim about once a year, because they do wear and that can cause problems.
Plus, you'll probably want to move to an 8 tooth rim, rather than a 7, depending on what size wood you'll be milling.
Let's see if I can find some rim diameters for you. A 404 x 7 rim is 1.52" OD. A 3/8 x 7 is 1.38" OD.
Any leads on an affordable bar ? What length are you wanting ?
I'm in the same boat, looking for a bar to fit the big Stihls, and not finding any screaming bargains on sprocket nose bars. Perhaps I'll go with a 36" Carlton that's on closeout at Baileys. It's set up with a 404 nose, but a 3/8 nose can be purchased separately and the overall price is still decent. I just gotta scrape together the money -- you know how that goes.