If the old motor runs fine - id run it as is - i ran my 80's agway with a 10HP briggs stock for years thru the swamp behind my folks place - it had its limitations ( tires being the biggie) .
Remove any low hanging pieces ( old deck hangars) - it sounds like its a horizontal shaft motor- so you shouldnt have any pulley issues.
A big thing is water/mud in the trans- you might want to put some fresh gear oil in, check the seals and check the oil ocassionally - depending how high it sits off the ground, you might want a front tire swap to a larger size, mainly for ground clearance.
You might also want to make a couple skid plates - depending where youre going with it.
It wouldnt hurt to make a rack or haul a trailer for some spare parts, gas , oil and even a cooler for those long distance trail rides.