A little different situation but one of our conditions of purchasing our home was getting a engineer to determine if an existing 70 foot long bridge on our property was structurally able to carry vehicle traffic as we wanted to build a four bay shop with an apartment on the other side. The cost for the report, 3200.00, a lot to put out before we purchased the home. Lucky for us it was cartified safe for passenger cars and had a reasonable earthquake load expectation, we bought and built the structure. It would of been a costly item if it had not been viable but worth it to us as we loved the property. (That was 20 years ago and we just had a bridge crew out to rebuild supports and replank at a cost of 45000.00, still love living here though!) Sometimes you just have to bite the bullet so to speak.

Your picture seems to show 2x4s, a doubled sill plate, a gap, then a header. If that header spans from an exterior wall to an exterior wall, the 2x4s and doubled sill plate aren't doing any work other than holding up drywall.

It is MUCH cheaper to have a few pieces of hardwood added (your professional flooring refinisher can do this for you...for a fee of course) and the whole thing refinished than it is to lay new plywood and then lay new hardwood. A full sand and refinish = $5/sf. That is the average cost. That includes labour and materials. If you need to source some pieces of hardwood and then lace them into the existing floor, it will add a few hundred dollars to the overall job. Now compare that to BAD (low end piece-of-junk) engineered hardwood. You can purchase the hardwood for $4.99/sf (please don't....they aren't worth the "savings") and then you will spend $2-$3/sf to install it...and that does NOT include new plywood (you can add another $2/sf for plywood+labour to install it). Refinish = $5/sf (plus a little more for new wood to be laced in) Add new engineered hardwood = $5 - $12/sf (floor itself) + labour ($2-$3/sf) + plywood.....a grand total of $9/sf - $17/sf. It's your call. The first thing you want to do is check to see how much wood is left on those old planks. Find a floor vent to look at the cross-section. Measure the amount of wood ABOVE the tongue. You will need 3mm of wood to get a full sand/refinish done. Start there. A sand/refinish is ALWAYS cheaper than new flooring. That's why solid hardwood is always more desirable. It can be CHEAPER to own over it's life time (60-80 years) than any other floor.

If you are in Florida, I could do it because I'm a licensed Building Contractor here, but I won't. I was surprised to find the only code enforcement on mobile homes here was for electrical only. Ironically, the worst undersized framing for the span I've ever seen was in a remodeled manufactured home.