Hi fellas, I just was able to purchase my first CAT. Its a D4 serial 7J102WS. Pony start. It has not been ran in quite some time. Id like to change filters and fluids before anything. Restored antique tractors all my life and know where to go for what on that subject. This is quite diffrent and really could use some help. Where can I find the correct fuel filters, oil etc ? I'm sure the pony will need a carb kit also. 
Would any of you be able to point me in the right direction to get me started?
Assume this will be a 6 volt system as well? Odd to me how the gen is bolted into the front cover lol. Basically not sure about any of this but willing to learn. Havent used a pony engine before etc etc. 

Welcome to the world of old yellow iron!! Looks like yours is a 1939 model and we would all like to see photos of different views of your latest venture. Some of the others will jump in here with the info you are seeking. There are several videos on "youtube" showing various pony motor repair and starting procedures. Welcome aboard
PS: If you give your location there may be someone near that can assist you from time to time.

Welcome! That tractor has stirred quite a bit of interest on social media. Nice bolt on street pads, long track frames and interesting dozer set up.
I think many (if they were quick enough) would have purchased it for just the pads alone for what we think you paid. Good catch!
I will say as some, obtaining the owners manuals are the second step. Preferably originals. Some reprints can be difficult to read.
Myself, I do not own any J series crawlers, but filters and parts aren't too difficult to obtain from dealer or your local parts house. Someone may chime in with the fuel filter #'s (You might actually have a brass oil filter that is re-usable/re-cleaned?)
It should be 6 volts for the pony starter, using a 12V battery won't hurt it unless you do a LOT of continuous cranking with it.
Keep us posted with plenty of photos and ask questions. Plenty of good help is available here. JM

Originally a Traxcavator I see

Figured it was quite a good deal. I jumped on it and the fella had this blade I got with it back in the woods.  Couldn't leave it behind. You guys are probably right on the buying price I didnt question it. 
 Assume the street pads are unusal to see / find? Quite thrilled to have this machine for sure. Had the pony running on happy gas last night thankfully. Do believe we may have a winner. 

One of the first things to do once you get it running is turn the tracks around since they are on backwards.

In regard to the tracks, I've been told by some oldtimers that they used to run the tracks this way on highlift tractors.

if you think the grousers look weird there not they are bolt on road shoes. designed so the grouser doesnt damage bitumen and concrete surfaces.
if your working it on dirt only take em off if it going to be a display only paraded machine leave them on. id still replace/ clean up and anti sieze all the bolts though so if you wanted they could be easily removed later plus it allows you to clean the rust and crud out of them.

not to many machines had them as wheeled tractors were more common for the work you would need road shoes for.

fuel and oil filters are still available but the original pn's have been superseded. talk to your local cat dealer if you have a parts book for your model they can cross match the older numbers. the air filter is just a oil bath drain wash it with kerosene or diesel takes same oil as main engine. oils are easy get a recommendation from your prefferd oil company.
note in the final drives and bevel gear housing you dont want zinc fortified oils as they have bronze gears and bushings and zinc reacts with the bronze so the tech rep for caltex told me.