I haven't done this... But I know a little about the vintage Nikons and the interchangeable screens for more modern cameras.
Many of the Nikon FM/FE/FA series cameras used the same, interchangeable focus screens... that apparently can be fitted to Canon 5D classic (but not necessarily 5DII since it uses a different focus screen from the classic). Here's a link to a website with more info about the Nikon screens and which models used them (most, but not all FM/FE had interchangeable screens.... some of the very late FA models use different screens, which may or may not work)
There were several different Nikon screens available. The standard "Type K" is most likely what you would want for manual focusing. It has a split image center dot with microdiaprism "donut" around it. You use the split image with shorter focal lengths. But it blacks out with longer telephotos, which is when you switch to using the microdiaprisms.
There are also "Type B" and "Type E" Nikon screens, both are matte focus screens only, the latter also has an alignment grid engraved. These are pretty similar to what's already in (or available for) the 5D classic.
There are also third party replacement screens available (see below), that have similar features in various combinations.
However, there is no free lunch... many of the alternative screens will have an effect on the camera's metering system. Canon has provision in a Custom Function for their own grid type and high precision screens... but not for third party screens.
You'll see the most problem with the very center part of the metering system, where the split image dot and microdiaprism "donut" are located. Your Spot Metering likely will not be usable at all. If using Evaluative Metering with the Center AF point active, that also will likely be more strongly skewed, though probably not as bad as Spot Metering. This will occur with either the Nikon screens or the third party... overall change in the amount of light transmission effects the general metering and any screen with central manual focus assist features will more stongly effect metering at that point.
Canon's own screens aren't terribly expensive, usually less than $50 US. If you are trying to do manual focus and using fairly large aperture lenses, the "S" screen might work for you (and won't negatively effect any of the metering modes).
The third party screens can be a lot more, most are $100 US and up.
The old Nikon screens might be found used fairly inexpensively. And there may be some other screens from vintage cameras that would work in 5DC or 5DII. Many vintage manual focus cameras used similar setup to the Nikon "K", by default. Since they all at essentially "full frame", the screens are bound to be close to the same size.
There are various modern third party screen manufacturers and sellers...
Haoda (no longer can find a home page for them, might only be avail. though retailers)
Also some cheaper Chinese made ones might be found on eBay.
You might already know... Any time you are handling or changing these screens you have to be very, very careful. The screen itself needs to be handled by the edges only. It's optical plastic, easily scratched or damaged and difficult to clean. And, the focus screen is directly above the camera's mirror... another delicate part that's very easily damaged (and expensive to have repaired... it is not a DIY job).