Oil tightness of helicoil -or insert- shouldn't be an issue as long as the o-ring or the crush washer covers the full hole. Thus it doesn't matter whether the oil goes through the plug threads or the insert threads.
Problem with it is that the drain plug hole has quite a big diameter, and buying the whole -your favourite brand here- set for just one use may not be cost-effective. It is a different matter if we are talking M6 or M8, where you have many chances to need them in the future.
I would install a new oil pan if there is one. If the threads are cut directly into the lower crankcase that's a different matter. If the oil plug is M10 or so, you can get a plug from a different bike at the upper size -M12- and cut new threads. If it is a big plug -like in the CB350-, something aroung M20, you could maybe drill and tap a smaller plug hole in the very same original plug, and jbweld the original one to the lower crankcase, using the smaller one for the oil changes and buy a cheap lower crankcase in the interim, waiting for the next time to remove the engine for the crankcase swap.
