Here is a way that could work if the chemicals fail...
... Cut all of the casting off as close to the tiller as possible. From here you have tow options. 1. Drill a hole as big as possible in the end cap. If it is at least 10 mm in diameter you should be able to insert the blade of a "puk" saw (small version of a hacksaw - the bade is about 7 mm high). Cut a (or even better more than one) slot into the casting without cutting into the tiller. Once the tension is gone you should be able to get the pieces out. 2. Take a big punch or a socket and drive the old casting about 5 cm deep into the tiller - ignore it there - and just install your end caps or better one of those Hobie 20 conversion kits. As an afterthought - or a third option - Heat might help as well. Just heat it up and let it cool down a few times ... that might work it loose. I would try option 3 first.

Patrick