I'm curious as to how you know it's a 1961 Diana yet you don't seem to know much of anything else about it? That's pretty specific info for somone who isn't sure what a Diana is.
The bike won't have a frame number to identify it, there would be no indication of year anywhere on the bike. Are you getting this from the foil tag (which is easily faked BTW)? Engine number?
Yes, a Diana is valuable compared to a Monza. How valuable? No idea, I've never seen one for sale. Less valuable then a Mach 1, more valuable than a Monza. Obviously it depends on condition. I'm guessing it wouldn't be valuable enough to warrant having a proper restoration done if you are just in it for the money. I'm also guessing you haven't got the resources to do a proper restoration yourself? No flame intended but you haven't given us anything to go on regarding your experience/skills with bikes in general.
IMHO it would make no sense to "cafe" one out at this stage of the game. What would you do to it anyway? It was already about as "cafe" as a single cylinder Ducati can get, right from the factory. If you start adding parts to make it look like every CB350 that's been "cafe'd" then it won't be worth any more than a Monza that's had the same treatment.
How about a little more background of what sort of shape it's in now, how you know it's an early Diana etc? Pics of the bike? Engine number? What about your background/skill set? Not knowing what you're working with, both from a starting point (the bike) and a skill set (you) make it rather hard for anyone to tell you what it will be worth when you are done with it.
If it is truly a Diana, and it's in relatively decent shape, your best bet is to not touch it. Sell it as is and find a Monza to play with.