Oven cleaner is not the safest thing to be using. I have found 'white spirit' works a treat and it only needs a 10mins of soaking and a wipe or brush over. As this is similar to the stuff in enamel it should do no harm to the plastic as long as you wash it off after. I suggest a good washing of the figure regardless of the stuff you use.
White spirit is a lot cheaper than the other chemicals.
All the best and please let us know how you get on.