How many calories do you think dirtbiking (moto) burns?

Hard moto burns a *kitten* load of calories. All I know is when I was riding I could eat and eat and eat some more and stayed lean.

You may be able to find better METS estimates, but a couple of sources are these:

https://golf.procon.org/met-values-for-800-activities/
https://sites.google.com/site/compendiumofphysicalactivities/Activity-Categories?authuser=0

METS are a research-based way of estimating average exercise energy requirements in a way that allows adjusting for body weight and duration.

One of those sites suggests that moto-cross (generic) is about 4.0 METS. Since I have zero relevant experience, I can't comment on the subjective plausibility of that. You can compare other METS values for things familiar to you to get a subjective impression. (Think averages over the whole time, not just peaks.)

You can plug a METS estimate and duration (with your body weight) into a calculator like this one to get a calorie estimate:

https://ergo.human.cornell.edu/MetsCaloriesCalculator/MetsCaloriesCalculator.htm

Be aware that METS estimates typically include the calories you'd burn just being alive and doing normal stuff in that time period, so you might want to back out your estimated BMR and MFP daily life activity level calories from a METS estimate. Numerically, that's not IMO massively important for a short duration high-ish METS activity, but it can be a big deal for long duration/low METS activities like slow walking.

The second site I linked above, if you go back to its home page, has extensive info you can access about what METS are and how they work for calorie estimates.

It's not an exact science, which shouldn't be surprising.

For exercise calorie estimates for calorie-counting, close enough to be workable is good enough, and as long as you periodically adjust calories based on real-world weight change results, consistent estimating methods are more important than exact ones.

If you have some statistics knowledge or applied math/estimating expertise, I'm guessing you'll understand where I'm coming from when I say that. (It's ballparking, basically. That can be good enough.)