Iv'e have a 2009 250 Evo 4t love it.my son has 2018 Factory 300 4T his is ridden every week -end.we never had a problem with eiither.He has just ordered 2020 Factory 300 4T .
These are the size and type as listed in parts list from Beta if it's the front 2 O Rings https://www.polymax.co.uk/o-ring-52-07mm-id-x-2-62mm-cs-fkm-viton-brown-75-sha
There's a Evo 200 on e bay but don't know what your budget is.https://www.ebay.co.uk/itm/2017-Beta-Evo-200-2t-Trials-Bike-Finance-UK-Delivery-available/293394506877?hash=item444face47d:g:sAAAAOSwxIleBLQH
Well looking in the parts book looks like they have sent you the 125/200 middle box which is shorter. no 11 mid box is shorter BT120126020 .Should be no 12 BT280126020