Wondering if you can help.

I got a Hangar 9 P-51 from a friend without a cowling and had to wait to get one as they are on backorder. I strapped a Supertigre G90 2 Stoke on the front and, as ugly at it was, I had to fly it. This thing hauls!!!

I finally got a cowling and the thing will fly for only 2-3 minutes and then significantly lose power, which makes for an early landing.

I think it may be overheating. I have what I think is enough ventilation with the exhaust hole being more than 3 times larger than in air inlet. ( See picture Below) I know this is more engine than specified (.60-.78), and possibly, the inlet is not designed to cool such an engine. But I also have read of larger engines (1.2)on this ARF in this forum.

Would it matter if my engine was mounted inverted and the head did not protrude out at all for cooling. The guy who had this before ran an inverted four stroke, but when he gave it to me, I didn't want to re-engineer the mount and the firewall for a side mounted engine. I had a 40 size mustang with an inverted engine and had no problems with it, so I figured I could do the same on the 60 size.

Any idea why it would only lose power with the cowling on.

Thanks