Well, at least you got it to run, so that's a start. Does sound a tad rich though.
Looks like a Cox Baby Bee. Not quite the same performance as the TD and Medallions but they'll still move a plane fine. I would say aim for an AUW of 12 ounces or less for anything you install it in, +/- maybe 2 or 3 ounces depending on the performance you hope to have. I believe these put out about 55 watts, so you can take that into account doing the math for what airplane you want to build for it. A really funny quirk about these engines is they have reed valve induction and so they will run perfectly happily regardless of the direction you start it in. This is useful as if you want to install the engine in a pusher application you don't have to buy special propellers - just put the prop on backwards and flip it the other way.
The fuel you are using is probably fine for it nitro wise. A lot of people say you have to run like 25% nitro in these to get good performance but really nitro in my opinion does less for more RPM and more for better tuning across a wider range of RPM like in a throttled engine. I have run Cox engines on FAI fuel (0% nitro) and they seem to do it just fine, but for ease of tuning I would still run maybe 10% nitro as the needle can become very sensitive with a complete absence of it. The difference in top end power is maybe a couple hundred RPM between 10 and 25%.
The thing you need to be careful of in these is the choice of oil. Cox engines really like castor oil - I sometimes blend my own fuel for them using only Klotz castor oil (though finding nitromethane that's not diluted here is not easy, so I have not done this in quite a long time) 20% oil content is the absolute minimum I would do - I usually mix or add some to about 22%. Most fuels branded for 1/2A use will have some component of synthetic oil blended in, and you want to avoid this. Often, fuel sold for larger control line engines contains the requisite amount of castor and can be used. Personally my favorite fuel to use is actually the Cox branded stuff - I don't think it's manufactured anymore so it's a little hard to find it NOS, but not only does it have 100% castor but it's sold in metal bottles so as long as they were never opened they can be old as hell and engines will still drink it down perfectly happily. I would shy away from running the car fuel you have right now, or at least check the oil content. Car fuel tends to have less oil in it.