If you want to do it right and replace the crank seals, esp the rear seal, you'll need to split the tractor and pull the whole engine out and put it in a stand to work on it. You'll likely want to have block hot tanked too while you are at it to clean it up good for new sleeves.
Engine kits are available from many sources including this site. Note though that most sites sell a 4 1/8 overbore kit that is really for WD45 engines as it is 6.5:1 compression (i.e. WD45) and not 7.25:1 (D17) and you will lose a few HP even though you get a few more cu in.
Only AGCO sells correct 4 inch engine kits with correct power crater pistons with correct compression ratios - 7:25 (D17, early E combine), 8:1 (170, later E & EIII combines) and 8:25:1 (175). Some sites sell and aftermarket kit the label as high compression by putting WD (4 inch stroke) pistons in it but they don't document the compression. AGCO kits are more expensive and do not include bearings (main, rod or cam) or valve parts and you have to buy those separately. It ends up being nearly twice as much from AGCO but it's the best stuff.
I just overhauled my D17 Series IV and used a 175 (8.25:1) AGCO kit and got my bearings cheaper from another vendor. Also - the AGCO gasket set in the AGCO engine kit is better than the aftermarket gasket sets in my opinion. More items and better gaskets (not that the aftermarket ones are bad).
I didn't have to re-do my head as it it been done about 100 engine hours prior (long story). Some of the non-AGCO kits do include the valve stuff too so if funds are an issue and you don't mind a few HP drop the aftermarket kit is a better deal.