Need ideas for replacing motor

[ZeroNthedark]

After nearly 650k miles, my 2005 CM870 is really starting to show its age. While I'm not opposed to just rebuilding it, it occurs to me that there may be better options.

Quite some time ago I believe Unilevers made mention of it being possible to procure parts to turn a 870 into a 871, but that it might not make the most sense. So this leads me to my main question-

Can a 871 be adapted into an older truck? I know finding a parts truck these days is not as easy as it used to be, but still possible. This will be going into a 379 Pete, so not too hard to find parts for, yet.

Other option that makes any sense to look into?

My truck is paid for, so if it's down for an extended period of time it is not a big deal. I am interested in any suggestions the members here might have.

Thank you very much.