Blimey, 70 miles per tank, that sounds extremely thirsty.
If you are to attempt an MCC trial you will definitely need to either carry fuel as per Ratty's comment or check your tank.
According to the Beta spec the tank holds 8.5 litres with 1.5 litres in reserve. 7 litres equates to 1.5 gallons.
The spec suggests a consumption of 3 litres/100 km which is a 94 mpg in old money. Now we all know that the manufacturers have Pinnochio noses when it comes to these claims so lets reduce that by 25% to a more realistic 70 mpg.
So at 1.5 gallons you should be getting around 100 miles before running onto reserve.
All the above of course assumes you don't ride flat out everywhere with the front wheel pawing the air after every corner

I would suggest draining the main fuel from the tank and then measure how much fuel there actually is in reserve.
It has not been unknown for the reserve tube in the fuel tap to be too long and holding back extra fuel.
If that is OK then drain the tank completely and check you can get 8.5 litres in it.
If that checks out OK then do a more accurate consumption check and if you are still only getting 50mpg I would suggest there might be something else wrong.
I have run older two strokes for years both on the road and in MCC trials and have rarely seen fuel consumption drop below 60 mpg.
I would have hoped that modern technology would have improved that situation somewhat but if your experience so far is correct then I can't see an XTrainer being on my short list despite it's other attractions.
Good luck and please let the forum know if you find any issues.