Call Dave Crosno's shop.  Dave has passed on, but his main man and Pam have continued the shop.  They're just about the only shop doing that kind of work.

It will entail sending in the rifle; it needs worked on in it's entirety, not just the trigger guard assembly.  It's a hard conversion to get done and make work reliably.  I've done one  and worked on a couple others.   

I'd like to take on making the parts for these and doing the conversions once I get a shop set up again.  But, that will be at least a couple years before I'll get to it, and I may lose the motivation before then.  Kenn Womack started in on this a couple years ago, but I think it got stalled when his parts maker couldn't follow through.  You might contact Kenn at rollingblockparts.com and see what his current status is on the single sets.

Another source of parts and changing to a single  set is using the parts Remington sold to Numrich after they stopped making the latest run of Rollers in the late 1990's! 
When I started my latest Rolling Block project I went to Numrich's site and found all the parts listed there to not only make a SST Roller, but also make it a pistol gripped frame with new parts! I am just finishing mine up, and sent everything out for color case and rust blue this morning. 
You might still want the good folks at Crossno's shop to put it together and get it working. But it might save a fair amount of money starting with new Rem. parts.

john king in kila mt has a set trigger conversion of his own design.   king machine . his work is great    art