keep the original A/C.. if its too ugly then change it.. or by a pallett of sweat rags.. your choice..
there are systems being designed for gas / diesel busses that are electric powered.. however they all have pretty large battery banks and a honkin alternator to run them while on the road..
Vanner here in columbus ohio makes such a system. i was lucky enough to tour and see up close their demonstrater "cool bus" system.. it is beautiful.. it employs a 500 amp alternator to charge up a Lithium ion battery bank.. its designed for busses that often sit idling just to keep the cabin cool.. it uses a variable speed DC compressor, EEV, variable speed condensors and interior fans and a big box of controllers.. I wont go into the technicals here (yes i got to learn alot of the details on said system)..
if the reason for not keeping your inside Bus A/C is because its ugly and takes up space then move the units.. or replace them.. I fail to understand why people remove working systems so they can sweat like its 1949.. but i can tell you my phone and emails blow up every summer with people asking me about hpw can they suppress the oppressive heat in their bus.. as their minisplit or camper A/C is like blowing on an ice cube in an auditorium... there are all kinds of options for interior units that likely are sized similar or will match your condensers and can fit in different places..
can you remove part of your origin al A/C cand keep part? if you are super insulating your bus.. floors, spray foaming the walls / ceiling, fixing up the typical heat dome of a doghouse (if the bus is front engine).. etc.. its very possible to get rid of 1 of the typical 2 systems and do quite well on the highway...
its also possible to install a high capacity dashboard A/C in a conventional bus and at least blow cold air on the front occupants.. (or assist your regular parked systems)...
this wont work for an FE / RE transit style (although there are dashboard evaporators made for bluebird and thomas transit busses)..
however it will work for conventionals.. as a hybrid approach.
for occupants in the back or on very hot days you make sure your parked-A/C (minisplit, rooftop, etc is runnable while driving... gives you extra Boost if the dash unit cannot handle it.
. the dash unit i used in ione of my vintage busses is an extra high capacity unit.. has dual blower motors (4 wheels).. and an evaporator coil that extends across the whole unit.. its rated at 36,000 BTU (think 3 minisplits).. uses a TxV along with a cycling switch.. feed it a #6 liquid line and come off of it with a #10 suction and use the exsting underbody condenser and compressor and even the lines.. you can buy burgaflex fitting to adapt the likely existing #12 suction and #8 liquid lines down to the #6 and #10 this unit takes..
I have no idea what the vanner system costs but it looks shiny and fancy and expensive..
get creative.. use what you have and build new along with it.. dont just rip stuff out.. thats about dumb..
P.S. i should add that dash unit is NOT the standard vintage air or Jeep / scout kit you find on amazon.. its actually sold as a ceiling mount Bus A/C unit.. I used it because its high capacity and it looks vintage for my 1978 bus..
P.S.S. I air-condition every bus i get my hands on... used to do the same when I built classic cars... a buddy of mine even takes it further.. he is into corvairs and air-conditions them