Can't find anything but dvdrips and a few SD quality webrips on jackett for anything older than the reboot series (post 2004 with names in the format of "Pingu in the City"/Mysterious helper/etc which have 1080p copies).
No immediate results searching to purchase blurays either which is a bad sign for hopes of 1080p copies to be floating around out there.
Remastering costs money so I would be surprised if they had done so and not tried to recoup it via a bluray release. On the other hand sometimes old content (particularly drawn) is sometimes simply upscaled to 1080p for streaming services with variable results (I've seen instances where pirate/fan upscales are far, far superior to the studio stuff released to streaming).
I also checked several private trackers including one dedicated to older cartoons and one for general older content and neither had anything but dvd-rips. According to wikipedia Amazon Prime has streaming rights in the US but I didn't see any amazon rips specifically of the older series so even if they have it in 1080p (questionable, you'd have to check the web page on amazon for the series for technical details) it isn't available.
Disclaimer: I'm not in any cabal trackers so I didn't check any of those. I also don't have that many dedicated non-English/European trackers in my jackett search so could have missed something.
IMO, good quality dvd-rips* at high quality (e.g. not re-encoded xvids done at 1000kbps) generally look acceptable on HDTVs and monitors. (Use mkvtoolnix to remove the Russian subtitle and audio tracks from the release I mentioned if you intend on keeping it long-term, use handbrake to convert any dvd vob files to mkvs, I suggest 4500kbps video bitrate as that's about the most you tend to see in dvds anyways)
*which can be found on public sites, like for example rutracker [dot] org.
Now if you have a 4K TV it becomes more noticeable at which point I guess I'd say it helps to have 1) a TV that has really good built-in upscaling hardware and software (not going to get this with most black friday $400 TVs sadly, you're paying near, above, or several thousand for a TV with this) AND/OR 2) a playback device (e.g. streaming) that has its own upscaling hardware and software (in this stuff like Nvidia Shield, Apple TV Pro's/4k's, and maybe the top end Xiaomi boxes are going to be best in class compared to cheap streaming sticks).