The Milbro Diana G36 is smaller than the Model 27 and is almost certainly an ‘updated’ Model 25 being of a similar size and power... a youth’s air rifle halfway between a junior rifle and a full-size adult rifle. There’s a possibility that a Meteor or a Falcon spring and possibly an ‘outer’ Relum Tornado spring could fit if a genuine Milbro Diana one is unavailable. 😎