Some context is required here.
Do you mean you're doing a game "after the fashion of" C64 games? Or do you literally mean you are developing a game for an emulator or even real, still functional hardware? You might find a hobbyist board where people still enjoy throwing together software for those (very) old systems. Personally I've written veeeeerrry rudimentary programs for C64 in their platform's built in BASIC interpreter (when I was a 'spring chicken') but my memories of specifics are nigh useless.
What "level editor" are you using?
You are asking for ways to mitigate the technical limitations of the Commodore 64 which are significant. The first bit of advice you'll see from probably everybody is this: don't make games for the Commodore 64. Unless you really enjoy wrestling with those technical limitations, which can be it's own kind of fun.
What you might do is this: instead of using two copies of the entire map (doubling the amount of memory you require for a map) you could instead limit the number of items that can be on the screen to some small number and keep each in an array, which in C64-ville you may have to manually manage yourself, but the code to do that will still be less than 2 kb. If you're picky and careful, anyway. You can see some methods by looking into how to manually manage arrays in C. The mechanics are similar, assuming my memories are correct and you can address memory on the C64; you don't have "Protected" memory but your game has nigh exclusive control so you just need to watch out for yourself.
Anyway, restrict it to maybe 10-20 items on the map in a FILO manually managed queue where each entry keeps track of coordinates and item code and maybe item code 0 is "I am not a valid item" for a safety net. ("Manually managed" means your code manages it, of course.)
I hope this is helpful.