I really liked Rat back in the day, 1961 through 1965. We flew 6-up right from the beginning with 100 MPH being the impossible dream. You really could throw together a RAT from parts around the house. But as speeds went up we went from 6 to 5 to 4 to 3 to 2-up.
My final rat, which still flys as a Formula 40 Speed (one-up) has a Series 70 K&B 40.
We were very happy with the ST G21/40 as rat engine. You could go maybe 105 to 115 and get good pit stops, plus it only cost about $30. The Harters pan was $3.95 and the Rodent or Raider kit (with pan) was $7.95. The K&B Series 70 would fit into the Harters pan OK. But then the roof caved-in for me and many others. It seemed like every six months a new "world beater" engine would come out and make everything obsolete. Worse yet, they wouldn't fit into the available pan, so getting a pan became a challenge.
We had 24 entries in a local contest in Detroit in 1963. Very soon there were only about a dozen people in Rat anywhere. The last time I looked, the winners were doing ten miles, including pit stops at averages speed above 120 MPH.
At this point in time there must be at least eight and maybe ten racing events that use the .15 (2.5 cc). That's far too many, but we never seem to drop an event.
The F2C results I read show that the winners are now able to complete 10 K races at average speeds above 120 MPH, using diesel 15's and with a lot of restrictions heaped upon them.
I would suggest that Rat be dropped, but it doesn't matter to me either way.