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Inkscape does have an isometric grid that's fully configurable. It could be that you just need to enable the grid visibility -- it might be there, but it's hidden. Go View menu > Page Grid. If that's the grid you need, you're good to go!
If it's only showing a rectangular grid, then open File menu > Docoument Properties > Grids and either hide or disable the rectangular grid. Then create a new Axonometric Grid. (As far as I know, axonomic and isometric is the same thing, as far as grids.) If that's the specific grid that you need, you're good to go!
If it's not the right configuration, then you can use one of the other AI files that shows the correct grid, and use that to configure a new grid.