6 months of 10-12 hour days... I would have my own replica arcade machine, suitably modified of course, and probably time to pan for the gold and jewels needed for the ring as well as forge it for that much work.
I do not know if you know assembly for something (does not need to be whatever ms pac man uses -- learn one and you do well at most, learn two and it is just a matter of learning a new instruction set most of the time) or general programming is like. Depending upon the people you have to help then this would be the stumbling block.
Assuming you do have a basic concepts of programming on lock
The game probably has some kind of loop when the fruit is on screen as I believe it only lasts for a limited time. I do not know what the coding will be, for the sake of easiness we will go some kind of high level theory coding.
The normal game would run something like
[ms pac man level]
at and of level goto "they meet" cutscene
at end of cutscene goto level 2
part of [ms pac man level] would have something like
[fruit on timer]
If fruit is touched (probably more like if pac man location = fruit location) then call sound effect, remove sprite, reset timer/set no more fruit flag, add bonus points, carry on with life.
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All you would do is then change the end of level to go back to level one. Change the if the fruit (now a ring if you find and alter the sprite) is touched to goto/jump/branch to cutscene and at the end of the cutscene you would set it to set whatever the emulator is watching for (a certain place in memory, a certain score value.....). If you had wanted it to play some massive custom animation and then some custom audio tune... well I probably tell you to look more at custom emulator coding and getting it load it as an avi and mp3, but if I had to do it in a game then yeah it would be a nightmare project.
If you did have a proper arcade machine then they often have some basic IO you could wire a transistor/relay to so as to power whatever you wanted.
Get me to do this on a system with tools that I know and I reckon I could have the hack done... it is 23:00 as I type this so possibly by 3am if it goes as such things usually do, then some high volume swearing as I fight with emulator code (though if I can do it on a PC with one of those nice Lua supporting emulators then it will be a breeze*) and just be after the morning rush hour to get me to hospital as I slice my fingers off owing to being the kind of idiot that operates power tools on no sleep. For an arcade ROM/board/architecture I do not know and having to go in cold then yeah it will be a weekend project for a couple of weekends, if only because the debug tools. If I had to also learn assembly in general... I don't know but a bit longer still and I would be tempted to drag in someone that does.
*
http://retroachievements.org/ (a site to add trophments to old games) already do something very similar -- they will pop up a message on screen in response to a game event (level cleared in certain time, 100 kills in a level, score over a certain amount), aka run code based upon the events of an emulated game which is all that is necessary to tickle the IO.