Honestly if the original is 50% it's intended size, it should look fine enlarged. Just do all your linework with the original, that is, if you do any linework to the enlarged version as finishing touches your pen lines will be significantly larger than the original. You don't want varying pen line widths. As long as your not enlarging it more than 100% of original, it shouldn't be a problem. I always draw a map at 100% scale or more of the final map, but that's just my preference.