First things first before getting carried away, is the motor powerful enough to use a dado stack effectively? I would say you need a 1.5kw/2hp absolute minimum for light cutting with a stack, 2.2kw/3hp is ideally needed to run it effectively. I can almost guarantee that you won’t be able to find a shaft aftermarket so that leaves having to have one machined for you by an engineering company which may cost a fair bit so you may want a char when you get a quote.
Don’t even consider some form of shaft extension as this is just a death wish.
I’d also wouldn’t bother getting a dado stack, I’d get a 6-15mm split groover set from CMT as they’re not much dearer and they’re a much better bit of tooling all-round.