That’s how mine is too. 
I recently bought some “oil bath air cleaner filter element” from Steiner Tractors. Product code is ABC442. It’s perfect and one whole packet fits perfectly in that compartment. It’s much better than the coarse wire wool I had tried before. I’ll post a picture when I’m back home. 
Cheers Jon. 

I worked on the Challenger this Saturday and got all the last parts sandblasted and primed other than the rear wheels. I took the radiator apart and broke off all but 1 stud. Hey not a total blow out. The core looks pretty good and the bottom tank is good but the top tank is shot. The shroud will need some work also. Once I get the studs replaced I will take it to the radiator shop and let him work his magic.

This weekend I started to take the lugs off the wheels. They are in bad shape and I was lucky enough to find a crate that had new ones in a shed. My question is it looks like they used a square nut type spacer to go down in the u of the rim. I am guessing they are drilled out to slide over the lug bolt. Does anyone know for sure?

Dan,

You have raised an interesting question here which I had never even thought about, yes I have seen the square nut spacer on some of my spade lug bolts on various tractors and always just thought someone had used bolts which were too long and put a larger size square nut over the thread to take up the slack, rather than a stack of flat washers, so never thought to look if the threads have been drilled out of the nut. 

Don't think I have ever seen them listed in the parts books but I may be wrong.

Malcolm.
 

None of my parts books show the rear wheels. 

Does anyone know where this goes? It is 201/2 wide and I thought it goes somewhere around the radiator area but it is to wide to go in between the U frame. 

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Don't know what your piece of tin is but it looks like the remains of a badly rotted out tool box? Can't think of anything else of the top of my mind.

Joe

Thanks Joe it is not the tool box and I am not sure it is part of the tractor it was just in one of the boxes of parts. See how much fun this project has been :)

Is it one vertical half of the fan shroud been flattened out?

Malcolm.

I know I am getting ahead of myself but I have some paint and decal questions. First all the paint codes I found on line for the green does not convert over to the newer paints. Anyone have a current paint code?
On the decals Maple Hunter makes a set for the Twin Power Challenger but not the Green. I see some tractors with the Massey Harris in front of the Challenger but mine did not have that as far as I could tell as there was very little to look at. Does anyone know how long that hood decal was and was it yellow or gold? Also did it have the big MH on the rear fenders? 
Then in the service manual I see what I think is a decal on the oil filter anyone know about that?
What about the starting tank and the water tank did they have the same decal as the 12/20 ?