Welcome vintagemodeltrainguy

I have the Engine, Omaha Car, Los Angeles Car and the Squaw Bonnet Observation Car.  I notice that there is a Denver Car, on Ebay.

If the motor is not running smoothly and you're not yet up to doing the maintenance and rebuild yourself, I highly recommend a full rebuild by someone fully up to speed. Let me know if you would like a referral.

Fastening the track down will only help if the pins are so loose they aren't making consistent contact and/or are working loose after running the trains. There *are* specialized track pliers available to properly tighten the pins in the track ends, but I've always made do with just a pair of plain or needle nose pliers. After first removing the pins and cleaning any corrosion out, just reinsert the pins and *gently* crimp them inside the tubular rails. After connecting the track sections together, finish by gently crimping the pins inside the rails they were inserted into.

And BTW, fastening the track directly on plywood will ensure you get the maximum train noise from your layout. Some do it, but there's a reason most of us use a top layer of homosote or extruded foam over the plywood, and/or cork or foam roadbed between the surface and the track, and that's done to deaden the sound. Nails or track screws *can* be used to hold the track in place, but they should not extend into the plywood, or the sound deadening can be short-circuited (and they're harder to remove!). 

Cork has been the 'traditional' roadbed, but has gotten a bit expensive lately I hear, and may be less available. I've been happy with Woodland Scenic's foam roadbed, and recently experimented with a similar product offered on eBay by sparks00033 (search on his name to get current offerings).

My go-to guy is Bob Testa, who goes by the handle "Marxmotorman" (appropriately enough!) and who has a YouTube channel featuring videos of his various projects, including one of a step-by-step rebuilding a Marx motor, which should give you a sense of what's involved. IMHO his rates are reasonable (he's more in it for the love of the hobby than mercantile reasons IME), and he tries to turn around projects as quickly as possible, unless he's badly backed up. Good luck!
 

I grew up running this MARX set as kid. We used to turn off the lights and watch the sparks fly off the tinplate cars as it ran around the track. 

Welcome vintagemodeltrainguy to the OGF. Alfred