First, you don't have a clue on what's going on here, so stop talking and start listening.
Chemicals alone don't increase endurance any more than they help you out with riding technique. They allow you to train more which helps you gain endurance capacity. You have to put in the training.
Red blood cells on their own allow your body to transport more oxygen. This does help your endurance (and recovery) capacity. Thick blood does not reduce blood flow, it increases blood pressure. When it gets high enough you develop left ventricle (heart) hypertrophy and/or a stroke. You damned well better care about your RBC, because it's important. Hydration does make a difference in terms of your hematocrit number, but in terms of a stroke, it may be a mixed bag. More hydration often means higher blood pressure, but less of a chance for accidental clotting. At the end of the day, a high RBC can be a serious problem. Aspirin does reduce clotting, but it's not going to show up on a blood test as a RBC reduction.
Here's the deal. You're getting arm pump because of shitty riding technique. Motocross is tough, but you've got to be relaxed while doing it. Some of the answer is getting your bike setup figured out, that will help. Spending money on a good mechanic/tuner could be a better investment than chemicals.
Overall, though, what you've got to get dialed is food and training. If your diet is absolutely on point, your training is smart and your equipment is right, then think about chemicals. When you're in the situation that your limiting factor is your bodies natural endocrine system, then you can give yourself a boost. There are no magic bullets.