No, it's not a stupid question as I brought that same one up over at the other site. Where the problem lies is that if you allow someone to swap in a bigger engine that they'll go overboard and drop in a high-performance mill. I suggested for those that blow engines (the Aquacraft .15 has a problem breaking cranks) that the replacement engines have a certain street price, say for instance, under $100.00 . This would keep people from dumping in a $300.00 .18 engine as a replacement.
The rules are going to have to be designed for use by the RTR monos and Cat's on the market also.
There will be other questions that will have to be answered. What constitutes mass-produced? Should there be a minimum amount of boats that have to be made or sold to say that it's a RTR boat?
We have to think of the long-term affects of a RTR class as right now we're basically discussing the under .21 hydro's but next will be the under .21 monos and cats. THEN we'll have to deal with the ones with .28's and .32's. What .28's you say? I can't say at the moment either right now.
Things would have been a lot easier if the manufacturers built boats to already existing classes instead of what I like to call the 'tweeners, too big for the "A" class but not really big enough to run in the "B" class competitively.