Try lunacy[1] it supports .sketch files and is quite good for the early stage it is in, sometimes it has strange behavior (e.g elements disappearing from the screen after dragging)and the navigation needs a bit of polish etc. hopefully they will get these issues ironed out soon in future updates. There are also online solutions like Figma[2] and Gravit Designer[3] which also has an electron based app for offline use. Not exactly a Sketch replacement but they will get the job done.

1. https://icons8.com/lunacy 2. https://www.figma.com/ 3. https://gravit.io/