About the movie itself and how to make roleplaying scenario out of it.
Retirement Home And The Mummy
This section might include minor spoilers!
Bruce Campbell is Elvis Presley, or so he claims. He lives in retirement house not in that good shape anymore. One of his fellow residents who claims to be JFK in black body thanks to goverment's plan, claims, that there is a great evil sucking old peoples souls to feed itself. An ancient Mummy!
So Elvis Presley and black JFK decide to fight this ancient evil.
Sounds weird, doesn't it? Yes it does. The movie was not that rediculous as it sounds, and definately in my opinion was not best Bruce Campbell movie out there, but it was rather entertaining otherwise. Best part in the movie is that you can actually make it a roleplaying game scenario for one or few sessions.
For the movie itself, I'd give 3 out of 5 stars. Entertaining yes, but nothing spectacular for viewing it again.
What Game To Use?
In this case I think it's mostly gamemaster's own opinion what rules he wants to use. But I have few suggestions based on my own collection I could use. I think, that more rules light game is better suitable than crunchy one.
World of Darkness Core: Storytelling rules are simple and quite versatile and they cover supernatural aspect pretty well.
All Flesh Must Be Eaten: Also pretty light rules to use, versatile character creation and you can quite easily create your Mummy horror with zombie creation rules.
Call of Cthulhu: Again, really simple and fast rules including insanity rules. You might want to ignore those though, as elder main characters might be senile and a bit crazy. And what fun is it to fight against evil Mummy if you go catatonic right after seeing it? CoC also is focused more on investigation than gunslinging what is a merit.
Those are only three first games came up on my mind out of my own games shelf, but basically you can use what ever rules you feel most suitable for the idea.
Characters are old and a bit crazy. You might want to reduce physical attributes to make characters have to rely more on mental problem resolution than just physical. Destroying Mummy should be more a puzzle than muscle.
Making characters individual and more outstanding from the crowd, feel free to allow players make their background as weird as possible! Elvis Presley who got tired of fame and replaced himself with Elvis impersonator for early retire. When this impersonator died, "Elvis" was dead. But real Elvis is still alive. JFK was not murdered, but his skin color was changed.
Use your imagination. I suggest that players choose some character from near history (around 60's to 70's) who might be dead, but for some reason is not really died as he's now in retirement home and concired as just a silly old fool.
Is character really Elvis, JFK or John Lennon or is he just crazy old fool? Does it matter? No.
You can use movie's plot about the mummy. Museum artifact what was dropped into a river during transportation incident and ancient seal broke open and let mummy free. Mummy needs souls to feed and chose old people. As soul sucking does kill the victim, old people are good victims as they are close to the age of dying more or less naturally. Oh, and mummy sucks souls out of butt. Don't ask.
Mummy should be powerful and not to be killed with straightforward power but some other trick. In the movie, the trick was fire. You might use it but if it does sound a bit too easy, make some other way to destroy it. But remember, in old a bit crazy people's retirement house obtaining source for big fire can be hard.
Obstacles of Old Mummy-Slayers
One problem is mobility. Character's aren't in the shape they were as young. They get exhausted, might have problems with mobility (stiff back, bad hip, weak legs, wheelchair), problems with health (heart problems, need for oxygen tank, second precis medication) and of course staff what won't necessarily like that old people wander outside or steal mummy-destroying items in night.
Another problem is that they are on their own. If they told to the staff that there's an evil mummy in lake what sucks old people's sould from their butt no one would believe them.
Character Death, I don't Think So
In my opinion this is just one-shot campaign (well, it might last few sessions but there is not necessarily any further continueity) so I advice GM to keep characters alive. It's not fun to die during small adventure with clear focus, is it? So, keep character's alive but don't tell it to players. If players know that they won't die, it is not as exciting to play. You might give some "Hero Points" what can keep them out of really dangerous situations.