I wouldn't recommend it, changing the 810 motherboard is known to result in a "PCI IRQ routing table error", see
this thread. In addition, anything newer than Pentium III will require a P4 connector, which your PPC board doesn't provide. With some trial and error you might be able to find a fitting board, get a P4 adapter and figure out the rear IO situation, but I see little point considering the scope works fine as it is.
Upgrading the CPU is more worthwhile, especially if you intend on using math waveforms, FFT etc. If you get a 1.2GHz or faster processor you'll be able to install the latest 1.2.1 version of the software.