Nice looking rifle, looks to be a pretty typical hardware store percussion lock from the 1850's/60's period. If it was a conversion I don't see any sign of it, but could be.
If it were me I would order a Track of the Wolf catalog. Track has full size pictures of all their locks and you should be able to use that to help you find a commercial lock that would be close. From that point you should be able to find someone (or do it yourself if your handy) who can fit a new mainspring to the lock. If all else fails a new mainspring could be made, but it would be cheaper to go the new spring route first.
The rest of the lock should be taken down and cleaned carefully, any wear to the other parts can be evaluated and dealt with accordingly at that time.