Mine did the same 2009 Evo model just had new seals fitted when i bought it Stood in the house 1 week later seals started leaking Replaced the seals and 2 trials later started leaking o/s first then near/side yes they were fitted correctly done many Beta's and other models never had any problems previously my son is a mechanic at a trials shop and said they have had a few Beta's done that when standing.
Should fit 94/5 used same mudguard have you a pic of the tank or without the mudguard.or its the wrong tank fitted.see pic of correct one. the tank you have looks like 96/7 in 98 went to alloy tank with the TX model
Checked all beta models with the USD forks not found any with the same fork guards or the 3 rings at the bottom of the legs. Stamped Paioli's wheel spindle needed tried several from different beta/aprilla models.
Well the 2003 model had a up and over front mudguard bracket,2004 on fork brace was straight across,between the fork legs.link shows you some differences http://www.motomerlin.co.uk/moto/motobeta.html
https://betausa.com/content/SUPPORT_PDF's/2016Evo3002towners manual.pdf
https://betausa.com/content/SUPPORT_PDF's/2016 Beta Evo 2-Stroke Engine Service Manual.pdf
https://betausa.com/content/SUPPORT_PDF's/2016 Evo 125-300 2-stroke & Factory.pdf
Don't know how different fittings are on the earlier model to yours.https://cdn.shptrn.com/media/mfg/9825/spec_file/174260963.pdf
Should be stamped on the crown of the piston,but can't see 1 on your pics,But what ever size you fit ,the barrel will need to be re nicosealed to suit.https://www.splatshop.co.uk/montesa-cota-315r-piston.html