I have previously used an 18R-C block to replace the one with a scored bore in an 18R-G, but everything else was swapped over. Motor went for about 160,000 km after that, and only needed to be re-ringed to be fresh as a daisy !!!
The only thing that caused a hassle is that the dipstick hole is on the other side of the block (meaning that the dipstick on the 18R-C block ends up being directly under the carbs when used in an 18R-G configuration).