At a 12/72t ratio that is 6.0:1 and to hit 45 you were spinning 6500 rpm, according to my gear ratio calculation using a ~14" tall tire that a 145/70-6 tire would come out to. Without knowing what strength valve springs you installed, I'd say at 6500 you're nailing the limits of the valve springs and/or torque output of the hp/engine. Anyway, the current question you run into is how fast you want to go and how fast you want to get there.
To give an idea of some different top ends, if you kept the 12t clutch and swapped to a 60t rear you would have a top end at 54 mph at 6500 rpm. 14/60 would go to 63 mph at 6500 rpm. You could make up some of the loss of acceleration by going to a higher gear ratio by using a torque converter instead of a clutch. You're probably going to want to find a kevlar belt if you try to use a 30 series converter instead of the stock belt it comes with though.
Anyway, at the end of the day, more teeth on the clutch and fewer teeth on the wheel will make more top speed. The big problem with more top speed, especially on a clutch, is that it will take longer to get there. You would likely not be terribly happy with the acceleration compared to current and would be looking into a torque convertor or a clutch that stalls over 3200 rpm to get a good kick when you get going.
To give an idea how slow it can get using a high gear ratio on a clutch. On my Fox with a 4.2:1 ratio setup, getting to 30 mph with a mild-built Briggs model 13 flathead takes ~10 seconds on a clutch that has a 2400 rpm stall. There is other considerations that go into clutch stall speed as well such as intended cruising speed, if the bike is being built for that.
And, as others have noted, rider weight can play into this as well.
Edit: To toss even more variables on there. The clutch itself can add to this. Some random Max Torque is going to have entirely different abilities in comparison to something like a Hilliard extreme duty or a Bully racing clutch. Not to say the Max Torque isn't perfectly fine for 99% of people, including myself, but it's going to have more slip compared to the $130 Hilliard or $300 racing clutch.