Bank-switching like the KimKlone does would be possible to support via a cartridge on the C64. Alternatively - though I don't think any of them have been pushed to that size, the CBM REU's did memory transfers fast enough that they actually were faster than using the CPU (because they could use every available bus-cycle for the actual data transfer).
So you would likely need custom/new hardware, but in terms of how, it's a solved problem.
That's all right, this is an event driven parser so at no point does it actually need a full representation of the document in addressable memory to parse it.
