The problem with inserting Lostech into a 3025 campaign is that most people of the time have no idea what Lostech really is -The Star League is a mythical, almost magical era and barely a memory to most, while the products of that time are also the stuff of myth and legend.
The reason why most of the 3025 era's 'Mech designs that date from the Star League are downtech is because nobody manufactures the components to maintain those original designs and almost nobody has an idea how to maintain Star League technology, so obtaining a pristine, SL 'Mech would be like a WW2 RAF unit getting their hands on a F-14 jet. Everything would be fine until you realize that for every hour of operation, the F-14 requires about four hours of maintenance by a trained crew.
The second, more serious complication would be ROM -the totally unknown organization that hunted down, destroyed and killed their way across the universe trying to keep Lostech out of non-Comstar hands. Finding a Star League cache would be very big news and ROM would soon be on the scene, to determine if they need to take immediate, covert action.
The one canon exception in 3025 would be the Arkab Legions, which, according to SB: Draconis Combine, were revealed to be fielding pristine, Star League era Battlemechs while fighting the Clans. No numbers were given, just a few lines pointing that out, as well as pointing out that the Combine and the Azami were separate realms, politically and were allies. It was my GM who pointed that out and suggested I play an Azami character in his campaign. He started out with a Mongoose, which is not an Earth-shattering 'Mech in combat but the Beagle Active Probe often proved very useful, especially in double-blind situations. It wasn't the Mongoose that proved to be valuable, but the character's servant and mechtech, who had trained all of his life to work on, maintain and repair Star League equipment.
In a practical sense, nobody could tell a Star League Lostech 'Mech from a downtech 'Mech from 3025 by just looking at them. Somebody would have to be paying very careful attention or have equipment to measure performance to tell the difference.