Furnace cold air source?

[caltrop18]

Hi everyone,

We just purchased a 1977 Golden West Sunny Brook. The furnace is an Armstrong MG66-110D2C, and I'm trying to figure out where it's supposed to get its cold air return from.

From all appearances, it seems to be made to draw air up from the space between the floor and the insulation under the house (see images below). The only registers I can find inside or outside the house are warm air supplies in the ceiling. The floors and some of the walls have been redone, but the previous tenants (who did the work) swear they didn't cover any vents. Is it normal for these furnaces to draw air from a space like this?

My wife and I are concerned that either we're going to go to need to use a lot of high quality filters or it's going to be unhealthy to use the furnace. Can anyone speak to this? Is there maybe an easy way to retrofit the system so it draws air either from the interior or gets it more directly from the outside?

View with furnace door open:

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View from space under furnace:

IMG_1345.JPG