If combine is failing, try selecting fewer surfaces to combine - they must have edges touching. If that still doesn't work, look for bad surfaces (red or blue lines) - you may need to remove these with the optimize functions (object menu).
You can use the Object->reduce function to reduce the detail although it's not predicatable and may not be suitable if your model is essentially 2D.
An alternative may be to retrace a new polygon/s over the outlines you have now.
If you can post or send a picture we may be able to give a more specific solution.
Andy
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