I didn't make major changes in the plot, though you'll need to adapt the setting a bit. The gladiator part needs some fleshing out, you'll find some info in the Messantia boxed set of course, but don't forget to check Signs & Portents #38 (Gladiators of Messantia) and #50 (Healers & Gladiators), both downladable for free from Mongoose site.
The dungeon part below the coliseum needs rewriting, though. I removed the monsters there and used the encounters below (I was using a homebrew version of Chaosium BRP back then, so I guess the stats won't help you much):
WHAT LURKS BELOW...
The cavern network is inhabited by the degenerate remnants of an ancient race. They generally live further below in the deeper caverns but will rush to the upper levels if they sense living food nearby.
Something else lives down there, something ancient and evil, an entity called Zoth that is worshipped as a god by the underground people.
The dwellers will stalk the PCs, waiting for the right moment to ambush them. They are not formidable fighters but they are so numerous that they can overwhelm almost any prey.
When the PCs are about to fall before the sheer number of their opponents, the dwellers of the dark seem to pause in the midst of their assault as if they were listening to something. A strange and entrancing tune can be heard through the tunnels. The sound is quite faint at first but grows louder with each passing moment. The dwellers will shout the name of Zoth and flee in total panic.
DWELLERS OF THE DARK
These degenerates are the descendants of an older civilized race. They moved underground after the great cataclysm but slowly regressed to a bestial state.
They retain some practical intelligence, they have a basic form of communication, an organized community and are able to use crude weapons and tools. They are carnivorous (and even cannibals) and will generally stalk their prey and try to overwhelm it by the force of numbers. They are humanoids with pale skin, no or little hair and bulging eyes.
They are not particularly brave and will run if they seem to be losing the battle. They fear fire and avoid bright light. Above all, they live in stark terror of the inhuman thing that lurks in the caverns and will run in panic if they sense it approaching.
Dweller Of The Dark
Improvised Weapon 35% 1D6
Brawl 40% 1D3
THE SPAWN OF THE ABYSS
“It seems to ondulate, varying by the second, rising gelatinously to height of perhaps three meters, spreading slimy tentacles forward. Its rubbery skin is coated with a foul ichor. The rancid stench becomes more overpowering with each moment. At the approximate center of this putrid black mass, a raw, slobbering hole, which seems to be a rudimentary mouth sucks in and out with obscene rhythm.”
Att- Crush or D3 pseudopods
Roll &Crush 80% 4D6
Pseudopod 60% 2D6
Hypnotic Song- POW vs POW or victim enters trance. Roll each round to break free.
Grapple- If at least two pseudopods hit the same target, The victim is grappled and brought to the creature mouth the next round (4d6 auto each round).
The bearer of the ring found in area 5 will be the primary target of Zoth’s attacks.