Before you spend £ots of money you need to establish the route from lens to audience. Others touched on this but raw footage can be a bit boring and there will be times when you'll realise there wasn't anything interesting in view.
Ideally pre-Covid19, I'd suggest joining a film-making club, where there are many folks with camcorders and too much advice . . esp. if you "don't know" - which is most of us at one time.
Audio is usually tricky, but I suspect motocross will be rather loud, so for the moment you only need to be sure the camcorder has an "Ext mic socket" - but wait until you gain some experience at the track-side. Get a filter-glass ( UV cheapo will do to protect the lens/shutter from grit/damp. and practice, practice . View your stuff on a large TV to judge if it's sharp, smooth and acceptable.
In some respects I suspect a cheapo "Action" camera would be better, since you can buy 2/or 3 and place them to get different viewpoints . . . the snag being you may not find anywhere to attach them and their wide-angle view can be rather uninteresting...unless you are attached to the bike.. OR close-to-the-action. They tend to be more robust than a camcorder, which could be unsuited to a "rough" situation. Their audio is usually poor, so a separate recording device may be in order. These are digital and record to SD Memory. A "Zoom" basic is about £80.
You will need to Edit the footage to make it interesting, and learning how to do it may take a while... there are plenty of Video Editors about and we all have our favourites...it comes down to cost... Perfectly good from £50 and you can pay over £50/month "Subscription" for the very best... but these can be tricky to learn ... better to go with one a friend uses as they can assist .
You'll need a reasonably good PC to run it, 64-bit with lots of Memory and pref at least two HDDs. 4K recording ups the cost and memory usage. HD may be acceptable( and is more forgiving.).. and probably easier to project.
Covid19 has blown all filming interest for me and I wonder that motocross is still in operation...
However, good luck with yr choice of gear.