I have two from that vintage, a somewhate newer 4.1 with infinitely variable motor speed and the on/off switch on the hose end, and a 3.9 that looks just like yours with the wood grain on the lid, pedal operated on/off switch and a two-speed motor, but the body is ivory color. The 3.9 doesn't have a Powermate......yet.
You can put a piece of Electrolux bulk filter material under the cheapie black open cell foam Sears uses as a secondary filter. That is what I do. For an exhaust filter I cut down a piece of Kenmore secondary filter material that comes in a 5X5 inch square for new Progressive and Elite Intuition canister vacs. Because of the way the body of these things are made, exhaust air only comes out of one place and you can put modern filter material in that place.
Finding hoses will be a problem. Central vac suppliers still sell ivory colored two wire hose stock, but finding one that will make you a seven foot long hose on your original Kenmore ends is challenging. That is one of my chores right now. Most only know central vacs and don't want to do anything today that is even slightly different than what they did yesterday. Sears stopped selling blank two wire hose and even SweetSweep cannot get it. I guess eBay will be your friend.
Ametek Lamb still manufactures a five wire, two speed 5.7 inch motor that will swap for the original if you ever wear it out and it cannot for some reason be rebuilt. My parents bought the 4.1 in 1982 and it has been used regularly since. It has never given us even the slightest problem in all those years. Only hoses and a Powermate motor ever failed. Take that Kirby!