The Vietnamese models, the Riva amongst them are not suitable for use as RC models. (even the ones stated as suitable, and/or sold with motors and rc fitted).
They are (there are exceptions and various manufacturers), nice display models, the finish is usually polyurethane, but they are constructed with tropical hardwood, and in some cases MDF. Hardwoods are very susceptable to shrinkage. They have no finish internally, untreated hardwood, and if gets water inside, the wood will expand and shrink.
The hardwood also means they are very heavy.
I have a number of them, and have disassembled one of them to gain some insight as to suitability for RC conversion, and concluded its not really a viable easy conversion. If they are all sealed (and this means pretty much complete disassembly and rebuild) then if the weight is allowed for, then maybe they would last, and be impressive on the water.
It pains me to say all that, as I have a close association with the country and travel there on a regular basis to visit friends and family.
They are designed and built for display only.
Hope this helps a little.
cheers
vnkiwi