a couple issues.
801 is a series.. means it is a 172ci engine built from 58-62 model years.
800 was a 172ci engine built from 55-57 model years.
the '6' in the 860 means it has a 5spd trans and live pto / dual clutch
that '801' .. you need to find the middle digit out on it's trans bellhousing to determine it's trans and pto setup.
4 is a 4spd, non live pto, single clutch
5 is a 5 spd, non live pto, single clutch
6 is a 5spd, live pto, dual clutch.
in 59/01 series the following were also added.. adn common
7 SOS 10spd full powershift trranny, ind pto
8, "" + ground speed pto / dual speed pto
not sure what your 860 needs.. but there will be some important differences from the 800 and an 801.. pinion spline differences.. fine / coarse.
brake cross shaft at the tranny..
clutch possibly different depending on model.
if the 801 is an sos.. LOTS of differences aft of the engine.
from the engine forward though.. pretty much the same..
if the 801 has a bad engine.. you could swap in your 860 engine..e tc..
post back some details.
what's wrong with the 860 for instance?