Here's a quick image to compare what 2X resolution Lemming graphics could look like.
Yeah, this looks pretty good

I'm a big fan of upscalers in general, they can work wonders. I've always loved the results of NL's upscaling algorithm, I've used it for a few different projects (the Lemminas style in Superlemmini uses it, for example). One thing that would be
very useful is a 150% upscaler; I'm not sure if one exists, I haven't been able to find one anyway.
This was done automatically by Topaz Gigapixel AI, set to 2X enhance.
Would this be 4x resolution, then? (i.e. if you've scaled up the hi-res sprites? Maybe I've misunderstood what's been done here).
Either way, whilst auto-generating the sprites through an art scaler like this is a tempting prospect, the subtlest of colour differences would mess up the spriteset recolouring functionality (required for neutrals, zombies, selected lems and alternate recolourings). It could absolutely work for a brand new standalone engine, though.
(Also, notice the colour of the lemming's back leg in the top frame; graphical aliasing like this would need to be manually edited.)
With that said, I've seriously considered the possibility of making the engine hi-res only and if I was to ever go down that route, it's very likely that I'd look at improving some of the graphics.