How “fast” are you and how much do you weigh?
I have an mxr and exr for my wife. I race in intermediate/expert and the mxr on a fast jumpy track in map 4 will last around 40 minutes before the warning battery low light comes on. I weigh ~200lbs. I charge with a 220v generator at the track so I am normally almost fully charge when doing 15-20 motos with a 20-30 min break between to charge.
However my wife will ride for an hour or all day at the track in map 2 or 3 and use 1/4 battery on a flat slower trails or track, she is a beginner. She weighs ~120lbs.
I do not think the mx vs mxr is noticeable in maps 1-3. I have owned a mx and our friend bought it from us. Main difference to me is in map 4 and suspension. However if you get the suspension revalved for you then it would be only map 4, and only “fast” people “need” map 4. I personally would prefer a map 3.5 haha. I like map 3 for low to mid but need the power of map 4 for the bigger jumps and long fast straights.
also I believe that If a Surron can make it, then an Alta can. Just don’t use map 4 and don’t go full throttle often for your first race. Wide open on the e bikes uses a lot of battery.
long story short, if you are very fast and weigh over 200lbs the Alta probably will not last an hour of racing, I believe.