We had similar 2x6 diagonal in our 1960 raised foundation ranch, however, the gaps in the floor were open to the crawl space and thus uninsulated, open to bugs, etc, so we laid our floors over.

What's under your planks?

First of all the floors are tongue and groove so there is no seeing thru except for gaps between the tips of the occasional odd plank and a knot in the pine that is gone. For those planks we will replace or fill. The key being that these planks are tongue and groove and even with the gap the tongue still blocks direct access to the crawl space. We are also considering spray foam insulation for additional insulation.

Live with it a while ... if it turns out to be a bad idea, you can always add hardwood.

There is a filler meant for fixing gaps in wood floors - it stays flexible. Trowel it on, scrape it off the boards, then sand, stain, and varnish.

I have finished subflooring for customers. It is definitely an option for you. Don't replace boards, as the new will not match the existing. Instead, repair and fill missing pieces with a wood epoxy fill material. Small knots can be filled with 3M D-100 floor epoxy repair material.

Best to live with the gaps and vacuum anything that settles between the planks. Large gaps can be filled, but I would recommend polyurethane backer rod first and then top that with a flexible acrylic caulk of the right color to match. Colored caulks for such work do exist, but you won't find them in the typical hardware store or big box retailer.

I notice several places where cuts were apparently made to lift a plank or two to place wiring or plumbing below. It looks like they did that carefully.

Some right materials combined with some skill and patience, can get you a great final result.