That is a nice tractor, it's got some wear, but once you figure out the repairs, sure looks like a keeper ! There was an R4 just like that in the machinery trader with someone posing next to it within the past year. Being an R4 and early in this series along with RD's and early D4's, it ought to hold some value, nice complete tractor and it was a gov't purchase orginally in O.D. green, little history to go along with it, probably surplused, most put in service in theatres of war never made it back, so it may have been used stateside or assigned to another gov't agency, prior to surplus.
I don't know much about these, that steering clutch pack, you need to see what the book shows on it, make a copy and enlarge it, I find this can be helpful. Many people suggest soaking with diesel, kerosene, a solvent that wont' leave and oil residue and working the clutch as per what was mentioned, though I think they can swell or really bond tight to where you have to disassemble them, seems the D2's and D4's of those eras were prone to this when left idle and not under cover. On the D2's I believe you have to get into the finals to get at them, I'm really not qualified to cover that area of one of these. See if you can look in the SRB and or parts manual to get an idea of what it looks like, hopefully someone can advise you further, and maybe it's not so seized that soaking and working it will free it up, beware though once freed up, could happen again might want to make sure it's covered up and run more frequently.
This and any undercarriage is really hard to judge from photos, though you can measure it up and see what is worn to what extent, would be real nice if you had one of those track gauges, or could borrow one to see what's what, real easy to use. The sprocket is not worn to sharp points, and the pin bosses are not hitting the bottom rollers from what you can see here, so maybe you can turn the pins and bushings ( read up on undercarriages, terminology etc. not hard to understand at all ).
www.tractorparts.com/undercarriage.htm
As the track chain slackens, the pin/bushing ride up further out of the root of the sprocket and wear it to sharp point if one does not do proper track maintenance to accomodate wear from useage as it wears. You can accelerate wear by not keeping it adjusted properly, having harsh operating habits, or not taking care of a loose track that requires more than an adjustment. If the track chain pin/bushing assembly is worn (outside only) you may be able to split the track, remove same, take to a track shop have em push out the pins and rotate P/B 180 deg to the unworn side, if there is one left. Internal wear is also a factor, not worth doing if the pins are worn out internally, might be best to install new pins and bushings, but only if the rails on the links are not wore beyond their limits, then it's time for a complete track chain. If the track links are not worn beyond allowable wear, and you have a turn left in those pin/bushings, and your bottom rollers, carrier rollers, front idler are all within wear limits a pin bushing turn would be worth doing, it will put the pin/bushing back into the root of the sprocket, and you'll be able to get some room back on that adjuster as the track will be tighter. Attached link might help with this part of it. Put it this way, the U/C does not look worn out, you may have a few moves left that will extend it's life well beyond the wear you will ever put on this tractor. Grousers are probably 50% worn by guessing from the photos. One thing is, you may have to work with what you have if there is nothing available for an R4 U/C, cross references over or can be modified to work like a D3 U/C for D2's. Might be a good idea to figure out your options if you plan to put some hours on it, see what can be done to extend the life of what's there.
That PCU, one thing to remember, when engaging the clutch to raise the blade, don't linger and slip the clutch, one steady motion and then apply the brake. Slippin those cones wil glaze em over from heat and or accelerate wear. It sounds like a brake/cltuch adjustment may do, but don't forget those bearing adjustments, would probably be the last time you ever have to deal with them, you don't want them sloppy and or misaligning the driven and driving cones, best case scenario, thoroughly check out that PCU when you have to time to go through it, for now brake and clutch adjstment might do, those cones have friction material, they are a wear item, not sure how hard it is to get the linings replaced in them, they are concentric, dog bowl shaped. Might be something simple like that spring, though mine cams over and locks the brake if you need to, the position before that, the spring may be important in relation to the adjustments, I made marks on my seat tank, per dimensions in the book to check the travel of the lever when adjusting.
Keep at er, nice one..... always wondered if these were cramped to operate, especially with the early fuel tank where it is, looks better than the ones with fender tanks, prior to the later seat tank models.