well for the .18 the run times will vary as all will vary. It depends on weather your heavy on the throttle or just do some full throttle blips for a secound or two. If your heavy on the throttle it will be a short run time. I never really timed my runs but a tank may (but this is a educated guess and may be off) last 10-15 minutes. I know that when I put my .15 that I almost got bored of how long it ran compared to the .18. I will time the a run tommorow if I remember. But I know with the .15 on it there wasnt the power that i had with the .18. But as for the .12cvr's run times I just dont know but I know it will be more then the .18.

As for any modding I have no idea what it will take to put a .18 in the rusty. And for the tank I think it would just be to big of a hassle. If your worried about run times then I would suggest getting the .12 so it would give you longer run times. But if you dont care about run times all that much then the .18 will be a good choice.

By the way I have a xxx-nt with the O.S.18cvr on it with a 75cc tank.