Twist the belt? I've never seen a PTO generator that was belt equipped.
I have a 12K PTO generator that I've used for backup power, its the one sold by Northern tools.
NorthStar PTO Generator — 13,000 Surge Watts, 12,000 Rated Watts, 24 HP Required | PTO Generators| Northern Tool + Equipment
I bought mine used a couple of years ago with a broken 3 point mounting frame for, as I recall, around 1K dollars. I repaired the frame and it works fine but after a few years I decided I'd rather have a dedicated generator so it's been sitting in the shop unused for a couple of years. PM me if you're interested, I'd be willing to sell it.
Here are my observations.
The good - no extra generator engines to maintain, minimal storage footprint when not in use, fairly easy to hookup, easy to use, gives you the ability to have power in remote locations. I've used a MIG welder a couple of times off the PTO generator in places where power wasn't available and it gives you a nice capability to repair things in-situ in the field.
The bad - using a 40hp tractor to produce a couple HP of power. Fuel economy isn't stellar compared to a smaller dedicated generator. Its also pretty loud in operation, the tractor is running at 2600RPM and because the gearbox in the PTO generator is a 3:1 step-up there is a fair amount of gear whine noise. Its not something I'd want to have running 20 yards from my house at night. If its going to be raining when you're using it (and since hurricanes are mostly when I use mine) you need to be able to park the tractor/generator in a semi-sheltered area when its operating. There may be other units that are more water-poof but the Northern Tools unit isn't rated to be outdoors in weather. It takes longer to bring the generator into service since you have to un-mount whatever you have currently have installed on the 3 point mount and remount the generator. Of course you can mount the generator before the storm hits based on weather forecasts....in my case this was always a sure-fire way to ensure that I never lost power in the storm. PTO's in operation are always a bit hazardous, I live in a fairly rural area but I was always slightly worried about some kid hearing my tractor running and coming over to investigate and getting his coat caught up in the PTO shaft. I'm sure a lawyer would have a field-day with this scenario.
I went the PTO direction because I wanted the ability to run A/C, I don't live in Louisiana so after a few year I decided that I'd give up that capability and go with a much smaller generator that can power my well pump, lights, refrigeration, ect.
I'm happy with my decision but if A/C was an absolute requirement I'd have stuck with the PTO set-up since generators powerful enough to reliable operate central A/C systems are pretty pricey.
I was able to operate one 4 ton central A/C unit fairly easily with my 12K PTO generator. The rule of thumb is 2HP per KW so your 40 HP tractor could power something in the 20KW range.