3 engines and one truck..... Pick one
Nice thing is, they will all fit. They are all the same block size so its really the internals that are different. To choose which one you want you need to make a decision... are you going to race or bash with the truck.
Nowadays, most .21 engines are over powering by them selves and powerful enough to run a truck as fast as you want. That's why most of the Pro guys use the .21's. But the plus side, is you get better run times with them than you do with the .28's.
If you are just messing around and not racing, put the .28 in there. If you plan to race, put one of the .21's in there. If they are still in good shape, they will be more than enough to be fast with and get that extra 2-3 minutes without having to pit as often.
I have herd the Axial motors are hit or miss. Hard to tune to run balls out and are not to good on run times. But I personally have not used one. All heresay.
Depending on which model the Nova and OS are, they would be a better choice all around.
Good luck.