The top pulley looks like the tensioner, but I'd like more close-up photos to make sure. VW used a similar method for belt tightening on the old air-cooled engines. The generator/alternator (depending on year model) had a pack of shims that was divided inside and outside the sheaves of the pulley. If you wanted more tension on the belt, you took off the nut on the shaft, removed the outer sheave, took out some shims, then reassembled with the shims on the outside under the bell washer. If a new belt was too tight, you put some of the shims between the two sheaves of the pulley. There was a fine line between destroying the generator bearings and a flapping belt.
There's no such thing as a chipper that isn't dangerous, unless it's one that won't run. That one looks pretty safe at the moment. The dangers can be mitigated (some) by wearing appropriate PPE and being very aware of the rotating hazards and flying debris. The FIRST modification I'd make to it is a guard for the belt/pulleys. Those are a little too open. Use some sheet metal or hardware cloth to keep the debris from the machine out of the belt. One good chunk in the sheaves, and you've got a broken belt, if not a bearing or sheave.
Don't let the nay-sayers sway your intent, but do educate yourself on that machine. What I see looks more like a shredder with the dual infeed hoppers. That's going to pulverize anything you put in it, not make chips for wood burning stove fuel. That's designed to make mulch, fine mulch. I bet you'll find flails on the flywheel and a baffle in the outlet chute. Removing both the flails and the baffle might net you some decent sized chips better suited for wood fuel, but typically the knives on a shredder are also designed to take off thinner cuts for the very purpose of shredding the material. Fine material is only good if you have forced draft combustion (like a bark boiler in a paper mill), otherwise, it just smolders.
As for the belt issue, the top pulley looks like the belt tensioner. It appears to actually be what is called a variable pitch sheave, which was actually designed for speed adjustment, but can also be used for just making the pulley look like it's larger, thus tightening the belt. The bolt arrangement suggests that to me. They look as if they're for adjusting the gap between the two halves of the pully, which effectively does the same thing as VW's belt tensioning design from the 1930's. It worked beautifully for 40+ years on every air-cooled VW engine. I've seen similar technique on other machines. If you take those pillow blocks loose for a new belt, you're going to be very disappointed/frustrated trying to get the bolts back in them. Think about it, every belt has to be tensioned. I see no vertical slots to move the two shafts closer together, so the only other way is to make one of the pulleys smaller in diameter. The old belt will be stretched and the cracks allow even more stretch. I'd guess that belt's probably at least an inch longer or more than it was new. Any decent automotive parts store can size that belt for you. Get the next size smaller to accommodate stretch of the new belt, but be prepared to go back up one size. Buy yourself a case of PB Blaster, WD-40, whatever your favorite penetrant is, and start working on that top pully to free it up. It'll be obvious how it works once you do. Noting the large pully on the primary shaft, measure that diameter, and then the diameter of the smaller one. Divide the larger number by the smaller, and it will give you the approximate 'gear' ratio of the speed increase. Most rear PTO's run about 540 RPM. That at a glance is about a 2-1 diameter, meaning the flywheel will turn about 1000 RPM. I'd do my best to stay within a couple hundred RPM of 1000 on the flywheel for best performance and safety. If there's no nameplate with ratings, better to err on the side caution. Find someone with a hand-held tach to check the speed, but be EXTREMELY cautious around that drive line. However, I wouldn't even dream of spinning that thing any faster than I could with my hands until I verified the condition of everything in it.
If you want to talk more about what to check and do to that machine, keep the thread open, because what I see is a machine that will be useful, even if not for what you actually wanted. If you're new to chippers, get LOTS of education about them. They're dangerous, and can't be anything but dangerous. You're gonna get your share of nay-sayers, but there are some old-timers in here that can probably remember much more primitive equipment than that, and give you very solid advice about the safety enhancements as well as performance of the machine. It can be run safely, but that doesn't make it less dangerous. Sounds like an oxymoron, but it actually says your safety is your responsibility. Take it very seriously.
