Send the gun to a very, very good sidelock gunsmith with a very small file. Really.
That said, Dan Lammers tuned the locks of one of my Smiths many years ago and a tuned lock is a thing of joy and wonder. He unfortunately left Phoenix and now doesn't work on Smiths.
And a heavy/creepy trigger pull in a sidelock suggests previous 'inexpert' tinkering which may well complicate the repair.

If it's a HOT it is much more complicated and there are very few Smiths able to repair the trigger.