Replacement Piston for Omer Airbalete.. mares?

You should replace the piston with the correct part for the gun. When pushed back in the gun the Mares piston tail is shaped to push the sear tooth down so that the sear lever will be depressed sufficiently for the mushroom head to pass over the top of the sear tooth and then be caught when the sear lever comes back up again, being driven upwards by the vertical coil spring mounted on the rear end of the Mares sear lever. The sear tooth and mushroom shaped tail abut against each other with flat faces perpendicular to the inner barrel axis, so the piston is caught as no torque operates to push the sear lever down again.

On the Omer "Airbalete" the pushed back piston tail rotates a yoke type sear lever (with a paired sear tooth and backing projection) until the sear lever is caught by the trigger operated slide that is pressed back onto the tail end of the sear lever by both a small horizontal spring and the gun's internal air pressure. The piston can escape the sear lever once the trigger is pulled because the sear tooth and mushroom shaped piston tail both have matching angled faces that cause the sear lever to rotate by directing a component of force perpendicular to the contact faces resulting from the pressure acting on the piston at some small distance offset from the sear lever pivot pin axis, causing the sear tooth to drop down and release the piston tail. So the basic trigger operation is very different, the Mares uses a stable sear lever which the trigger operation tips over and the Omer "Airbalete" uses an unstable sear lever which the trigger operation releases from a lock condition.

Although a piston tail could be modified by machining metal away it is much safer to use the replacement part which has the required strength. By matching the contact faces the wear is reduced, so while a mismatch may work for a while it will cause problems because all the force will be concentrated at one point on the sear tooth and mushroom tail contact faces.