So, I'm about to start a campaign and I have a Compound Bow at my place that we could get access to. However, there are no stats for them in the Weapons section. I understand this is meant to be simplistic and you can use a "counts as" guesstimate. I'm just trying to see what others might think works best for these weapons.

There is a "Hunting Bow" in the Pandemic (Post Apocalyptic) section. But it doesn't cover the broad number of types: Longbow, Recurve, and Compound. And there is also the Crossbow. Here are my thoughts on the types and what Weapons values they should have:

Longbows: No Dice Added +3 Damage
These are the most difficult bows to use and so shouldn't add any dice to the pool. But, if you have the correct weight for your size, they shouldn't add any negative dice either.

Recurve Bows: One Positive Die +3 Damage
The recurve on these bows make it easier to draw the bow and, as such, allow a person to use the same draw weight as the Longbow without trouble. Adding one positive die to the pool.

Compound Bows: One Positive Die (Possibly two if customized?) +4 Damage

These are the best bows out there for both precision and weight. They allow even heavier draw weights to be used and allow the user to hold the draw easier as well.

Crossbow: Two Positive Die +4 Damage

These were designed to be easier to use (since the user can keep the crossbow ready to fire without having to hold it drawn) and were designed to be armor piercing. The downside, of course, is the reload speed. Possibly needing a "Major Task" to reload.

What do you think? Is there a guide out already for these? If so, where can I find that?

I like it! I was wondering the same thing as well. But I wouldnt make a distinction between long and recurve bows. Of course I know nothing about bows, and they seem to me to have the same function....

What is the range difference from bows to crossbows? Which is more accurate at long range? I almost think that a bow can hit pretty hard at range if you arch the shot enough. And it seems to me that a crossbow would be more difficult to "arch".

Of course, all bows have the same "function". It is the features that make a difference. It is harder to use a Longbow since you have to pull against the full weight of the bow the entire time you are holding it drawn. This can lead to a little shakiness for those not used to using them on a regular basic. Whereas the recurve bows reduce that a fair amount, making them easier to aim without having to pull too hard at the same time...

The crossbow does lose more speed and accuracy at longer range due to the bolts size as opposed to the size of arrows used with bows. So, you could do something like adding two dice for shorter range and one die at longer range for the crossbow. Whereas the bows do not lose dice (except in the case of extreme range)...

I'm no expert on archery (sounds like the OP knows a lot more than I do), but I always thought the great advantage of a compound bow was that it somehow reacted to the user's own ability to pull back the draw string. As such, I'd be inclined to give it variable damage based on the strength of the user. Leave the dice at +1 or even +0 to prevent the thing from having too much going for it.