Parallel ports don't have a speed - they toggle status pins one byte at a time.
The problem might be that the software is looking for an LPT: port rather than the emulated PRT: port.
Capacitor replacement could be done by most any computer repair shop and they would likely be able to test the serial and parallel interfaces on the plotter first to be sure.
Roland is supposedly compatible with HPGL; this wasn't completely standardized until HPGL/2 was released with different devices obeying various subsets of HPGL However I expect PUPD, et al will indicate the commands are being received.
Direct access to LPT: was required for time-sensitive operations, such as for CNC machining; keeping ahead of a pen plotter would not require that level of communication.