Tyres will need to get replaced somewhere between 4-6 race meets I believe. They should last anywhere between a month to 1.5 months if you only use them for racing, otherwise if you use them while practising, they will obviously wear out faster that way since you are using them more often - as you would clearly see.
As for engines, when you get a new engine, you should replace the conrod after you finish breaking it in plus another 0.5lt-1/8 of a gallon on top to be sure. Then once you have put in the new conrod, you should run the engine as if you are still breaking it in, because you still have to run in that new rod you just put in. Some don't even bother with replacing the first rod many litres/gallons after break-in and it runs fine. But it's better to change it within the first 4 litres/1 gallon to be on the safe side - it's better to spend the extra $40 for a rod and working engine than to risk losing $400 or more!
With the piston and sleeve, you can't change only one of them in ringless piston engines such as these, you can in a 4-stroke with rings, but these engines have matched pistons to the sleeves they are in, so you can only buy them as a set. The only time you would change these is when:
a) something goes wrong and it won't run any more.
b) it gets worn out, won't hold its sompression any more and is hard to keep running.
c) it breaks!
Then you have to run in the whole engine once again as if it were like new.
I think for cars like the Evolva M3, you should run the engines on 20%, compared to 16% for 1/10 onroad. Im not sure on what would be better for 1/8 as I don't race that class (yet
) but I'm sure somebody on these forums would know for certain.