Jieboy, I'd say you're gonna find it very difficult to mount an auxillary engine on a Valiant 490. I used have one with a Merc 75 (probably about the same size as your Yam 4stroker) and we just couldn't fit one, we even had to get the A-frame custom made as the frame Valiant were supplying was too narrow to accomodate the main engine. If yours is the same as mine with the transom knees either side and the black tube straps, then it's gonna be very awkward. Good luck with the search though!!
The small Valiants are great fun, especially when you put a bit of power behind them!!
Current engine setup allows the fitting of the aux direct to the transom. It sits there very solidly, has never budged even ploughing home for a pint in the middle of an F8!!
I somehow doubt I'd still have it if it was on a bracket, just something to think about.