Original / Diana Mod 5.. Has Anyone Fitted a Tian Spring?

[D-Man]

My Origina Model 5 recently went through some total power loss.. Yesterday, I stripped it down, and found the central ring of the piston seal had partially disintergrated and lost it's place on the piston ( new seal ordered )

So then I got thinking about the mainspring ( it being somewhere around 50 years of age ).. a quick look on the Bay of E threw up a Titan item that fits the model 25 and 5. The size stated measures 15mm longer than the one I removed from the gun, Now, I understand that I may need to shorten the new spring.. BUT! Is the original spring likely to have lost 15mm over 50 or so years? or will the new one compress too tightly to cock the pistol? or should I make the new spring slightly shorter than the origins one due to it being a stronger one?

Has anyone done this upgrade? I'm already equipped with Solvol polish and Bumslide grease.