You're also in prime capacitor plague territory ... I've had very mixed luck sourcing industrial PC components from Aliexpress / eBay sellers, the last go-round took a few months and 3 exchanges to get a working board.
For the money, time, and risk required to buy a bare mainboard you can buy an entire "premium" late Windows 98 Pentium 4 based computer (this one has RDRAM for a particularly interesting bit of period correct nostalgia): https://www.ebay.com/itm/304268855967
I understand the appeal of a single computer doing it all, but the SB16 was not an objectively great sound card outside of it's period correct golden age (and many would argue, not even then). The effort and cost to drag it forward into a more modern Windows 98 compatible machine while missing out on the 3D positional effects and other features of more modern PCI sound cards that may have slightly poorer DOS support but vastly superior Windows features seems impractical, from an objective standpoint.
By the time you source a CPU and cooler to go with that mainboard you'll be at close to $200, plus you still need to find an AGP graphics card. You also don't know if that Diebold ATM motherboard supports the sorts of features you're going to want as an enthusiast, the BIOS may be heavily stripped down or limited unlike a consumer/commercial mainboard.
If you want faster storage what about a simple PATA-SATA adapter and a small SSD hard drive in your current machine?