Rebuild is to take your exsisting power plant and either overhaul (to replace only the parts that are worn out) or rebuild (replace everything except the good castings back to new factory spec) it so to have a fresh power plant that hopefully won't give you problems for many years. The typical 2 stroke Detroit Diesel can be expected to last about 500,000mi with proper care.
Repower is taking your exsisting power plant and completely changing it to a different engine. Like taking the 6-71N and 4 speed manual out of the 4104 and replacing it with a 6V-92TA with a V730 Allison automatic. Or taking a 8V-71N and the 4 speed manual out of the MCI8 and replacing it with a Detroit Diesel Series 50 and 6 speed Allison world automatic transmission (about the most fuel efficient engine available now-although it is out of production). Many, many different engines that are possible to change your bus over to. You can stay with some of the mechanically injected engines like Cummins's C8.3, L10, NTC, KT; Detroit's 8V-71, 12V-71, 6V-92, 8V-92; Caterpillar's 3208, 3306, 3406, 3408; International's DT466, Mack's engines. Or you can go through the serious job of converting the powerplant over to a modern electronically controlled engine like Cummins's ISC, ISL,M11, ISM, N14, ISX; Detroit's Series 50, 55, 60; Caterpillar's 3126, C7, C9, 3176, C10, C11, C12, C13, 3406E, C15, C16; International's DT466, DT530, DT540; Mack or Volvo; Mercedes-Benz 900 or 4000 engines. Many choices-my opinion, just stay with the mechanically injected engines-most are very reliable and won't present the tremendous change over problems an electronic engine will give. Good Luck, TomC