I'm no expert, but I would imagine that Nim is capable of producing code suitable for modern 8-bit compilers if it is able to do so for the Tiny C Compiler.
Following the C89 standard would be best for my use case, but most of C99 is usually supported too. As I intend to create text adventures, 8-bit integers should be enough in most cases and using 16-bit integers shouldn't slow things down too much. As long as the programs run faster than they would in BASIC, most players would be happy.
