I recently purchased a single story ranch style modular home built in 1969. Interior dimensions are about 22 1/2 x 47 feet. The roof is 4/12 pitch. It is located in upstate NY.

In the living room/dining area there is a 21′ 5″ open span which sags about 2 3/4″ in the center. This appears to be original construction, neither myself or the inspector I hired could find any evidence that there was once a load bearing wall, support post etc. There is a main “beam” running down the center of this space (see image) consisting only of 2 2×4’s on edge. This beam is in the center of the house running perpendicular to the trusses.  I believe this is where the 2 halves of the house were connected.  The smaller beam to the left of the main one is one of many decorative box beams that I am in the process of removing.  

The inspector was confident that it probably wouldn’t get much worse over time given that it’s been around for 50 years and is still solid. I don’t, however, like the look of the sag and am worried that If I were to sell the house it might scare away potential buyers.  I also want to make sure that it doesn’t get any worse.

I have come up with a potential fix – there is a good, convenient spot to add a support post which would almost split the span in half (10′ on one side, 11′ 5″ on the other). There is a solid support post in the crawlspace directly below this spot as well.
My question is: would it be possible to replace the beam with something sturdier like an lvl and jack up the 2 3/4″ sag without breaking/cracking anything above? I would then use the support post to transfer the weight down to foundation. OR would it be better/safer to leave things as they are, add the post so it doesn’t get any worse and maybe reinforce the existing beam. I think I could box it In and simply make the sag less noticeable.

The post shown in the picture is just something temporary I put in.

Any suggestions would be greatly appreciated!