i'd contact the chemistry dept of your local university and ask for advice, or even wander over there, poke your head into labs and try to find a grad student who might help, they're interested in oddball stuff.

check out their histories first if possible, might be someone involved in pd, etc.
food science might be another dept.

regardless, you need to measure a known levodopa source to verify your home grown technique if you try to devise one, researchers try to find a pure qty of a substance to act as a control, if they get that right, you know you're procedure s accurrate, but even then you have to repeat it many times.

i can understand why it would be useful to measure fava bean tincture, even generic sinemet batches to see how consistant they are but can't quite understand banana skins, lots of herbicides/pesticides used on bananas and if the world reaches a point where we can't depend on getting sinemet we likely won't be getting bananas.