S-video is just black and white composite plus color data on a separate wire, the laziest option is to just combine them back together (can be done with a single capacitor) and feeding that to a disposable-grade composite to hdmi converter
Similiarly it can be split into RGB or YPbPr by a NTSC decoding chip (that's what goes on inside any 100%-analog-video-processing display), not many affordable commercial products for this but you can find online schematics involving a dedicated chip and little more if you wanted to build one yourself
If you want a (probably) decent premade solution, there's the Retrotink... at a price!
