black engine oil is good - it means it did a jolly good job of mopping the engine internals and as this engine does not have a real oil filter just an excuse in the shape of a metal gauze - its pretty normal to drain nasty oil from this engine.
The best sump gaskets are the handmade thin green ones. cheap and cheerful. in PIB colony Karachi there are a few dozen shops who deal in only gasket making. And nearly 2 of them carry such maintenance gaskets by the bulk. Ask a karachi member to buy and send you some.
Dont you love the oilbath air filter. a filter change costs less than nothing 