I see a lot of really complete frameworks being promoted for this jam. Anyone else spending all their time on coding and essentially no time on graphics because there won't be any time left? :D
Anywho, to get nice upscaling in sdl2 set your screen dimensions to a nice size and use this to upscale:
SDL_SetHint(SDL_HINT_RENDER_SCALE_QUALITY, "nearest"); // To get nice 'pixely' upscaling
SDL_RenderSerLogicalSize(renderer, 64, 64); // Upscale to current screen size from 64x64