I can more than likely do it. I TIG all the bolts on the M700's we blueprint.
I have a guy showing up tomorrow morning at 0600 for a little welding side job in fact. He needs a few tabs stitched onto the frame of his new Pro Stock Drag Bike.
I've not done a Marlin before, but I can lay an itty bitty bead pretty well with my Lincoln.
Just know this:
There's no certainties in this. IF the bolt handle/body has ONLY been soldered and NOBODY prior has taken a TIG/MIG/STICK torch to this thing then it should go well.
IF the bolt has had an electric torch put to it and the surfaces weren't properly prepared then I'm afraid it's likely junk. They (torch) get so hot that any residual solder instantly contaminates the weld and it destroys the usable material. It basically turns the steel into garbage pot metal. ALL/ANY trace of solder MUST be removed prior to TIG welding it. Otherwise it'll just break again and it'll be ruined.
FWIW, not too long ago I worked closely with a guy who welded up reproduction vintage H/D motorcycle frames. Were talking 09's to 15's. The original frames were soldered between the tubes/castings. When original bikes would come in with cracks it was always a challenge/crapshoot as to whether some hamburger went after it in a previous life with an electric torch. If they did we had a WHOLE LOT of work to do to save it. It could easily result in a complete tear down and replacement of the casting/tube where they attached.
These are bikes valued at over a 1/2 million dollars. You only get one shot at it. "Competition Distributing" in Sturgis, SD. It's where I got my business started while waiting on my new building.
Good luck, be happy to help.
Chad