What is the biggest engine i could fit on my hsp gladiator?

I have a hsp gladiator and i wanted to get a bigger engine for it, at the moment it has a vx-18 engine it. Could i have a bigger one in, if so what mods would have to be done.|||You would need to make sure the mounts were correct and that there is sufficient body clearence; but, WHY would you want a bigger engine as this will NOT increase the speed capability since they would both turn about the same rpm.