Is the sony a7 II good for xc and mx photos?

Better for you that it's a D90.  It's faster and 12MP instead of 6MP.  Have someone set it up for you who knows Nikon.  Is there an "AV" and "TV" on your top dial?  

At 75mm the aperture is 4.0 but as you zoom it goes to 5.6, in lamens terms.

As a lot of people have said though, just get what you can afford and go shoot. Even the best setups will do nothing for someone who doesn't really get out there. Aperture isn't as important since most moto is during the middle of the day, so you'll have to stop that down anyway. Keep the shutter speed above 250 or so (minimum) and fire away. I shoot on a Sony a6000 and it works perfectly for me. As you improve, get better glass. Focus on finding good compositions (I like corner shots better than jump shots) and let it rip. Pro shooters absolutely have great opinions but really for a beginner the differences are negligible.

He'll have M, A, S, and P on that dial as that's Nikon's version. Manual for the D90 is here.

My dad used to shoot on D90 and loved it. It's a neat little camera.

I see you’ve already found the Nikon, but to anyone else considering an a7ii, don’t. The batteries for those are garbage. That’s an important thing nobody else mentioned about the a7ii.