Old one should just be retained by a circlip inside the cap. But then again if it's been home modified, who knows.

I'm not sure if you said you had the old cable or not?  If you do, check it closely as the missing piece might be threaded onto it. 

I have a lot of odd carb parts...  the snap ring cable elbows seem to be OEM parts only.  I will look and see if I have one in my parts bin.
 

Niche Cycle out of Florida sells many different sizes & styles of cable guides but most seem to thread in and are held in place with a jam nut, maybe it’s possible to tap threads into your existing carb cap? They also sell threaded carb caps but your existing cap is the “tall style” which are not nearly as common as the short caps,

The bike came with a couple throttle cables, neither of which were OEM. I looked in the bin of parts I have, I don't think the other pieces of the linkage are there. I thought about buying a cheap chinese carb and cannibalizing it for some parts, but it seems like all of those have straight linkages. I could probably just tap the original part and install one of the threaded guides. I think that's what I will end up doing, it seems like the original part is pretty impossible to find.
On the plus side, it looks like the Evapo-rust did a pretty good job cleaning up the tank. There's still some loose tank sealant stuck in there, but I don't think it will end up being a problem. Pictures to come!

There is an original one on ebay, comes with carb cap. It's about $40 after shipping but sure would save a lot of time and hassle. Let me know if you want it and I'll PM you a link.

Yeah, that would probably be best. Shoot me the link and I'll pick it up. Thanks!

Darn..  It got sold but I found another one that comes with cables for $34 shipped.  I'll PM you the link.

Link sent to your PM.  Let me know if you don't get it...  hopefully I didn't give enough info out that someone else grabs this one!  LOL.

A lot of people browse this site and I've seen parts linked here and someone grabs it before the intended person gets a chance.

Just bought it. Thank you!

Nice big update on the DT250 today!
Thanks to MarkT for the link to the carburetor part on eBay!

I ordered a used oil tank from a 1974 DT250 that I found on eBay. It was one of the cheapest I saw on eBay, and it came with the original oil cap. It's pretty beat up, but it holds oil and the sight glass is intact. It's not the same year as the bike, but it looks close enough. The tank: imgur.com/a/XLnMc7q
The throttle cable/oiler cable is installed now, and yesterday I spent a good deal of time working on the oil pump to get it to work. I still haven't tested the amount of output to see if it's within specs, but it does pump oil now!
The pump: imgur.com/a/Bp425Gf
The cable setup and routing: imgur.com/a/E66GH8x
I also installed the front brake cable, and thankfully it seems like the front brake works fine. I have a sneaking suspicion that the front shocks are shot, but I haven't confirmed it yet.

As with everything on this bike, it's one step forward and two steps back. First off, it sounds like the crank bearings are about to die when it's running. Not entirely surprising, I removed a lot of water from the crankcase when I got it. Secondly, the clutch appears to be seized. When the engine is running, I can put it in gear but it immediately lurches forward and stalls. Again, not super surprising since the gearbox was also filled with water. I guess that I will have to pull the clutch and rebuild it, but for now I need to get it running better. It starts pretty easily but it usually dies out after a few seconds. I think it will help to get the real fuel tank reinstalled. Feeding it from a plastic water bottle with a hole in the cap isn't probably going to make it run the best.
I still haven't sorted out the seat, but I think I will make something up out of plywood and craft store seat foam at least temporarily.
The evapo-rust seems to have done a decent job on the gas tank. There's still a bunch of leftover gas tank sealant sticking to the sides of the tank, but I think that's just going to have to stay there for now. Once I get my borescope camera working I will post some pictures of the inside of the tank.
That's about it for now! I don't know how much hope I really have for this engine. I suspect that it sat with water inside the engine for a decade at least. If I can get it to run and ride, I will most likely sell it or trade it. Getting the oil pump working was a huge step in the right direction, but there's still a lot that needs to be done.