I would try a 5751 rather than a 12AY7.
Firstly, very low gain valves like the 12AY7 can cause over dissipation in the plate load resistors which could compromise reliability. I know this as I've measured the actual dissipation in several amps. A 5751 draws about the same current as a 12AX7 so will be fine.
Secondly, 12AX7s etc contain two triodes. These are cascaded in V1 of the Classic 50 so any reductions in gain are multiplied, so you don't always need to use a much lower gain valve such as a 12AY7 to have a very significant effect on the gain structure of your amp.
Thirdly, the biasing of the pre-amp stages has been optimized for use with 12AX7s. Using a valve with radically different characteristics may mean that the valve is not optimally biased and my distort unpleasantly.
Using only one speaker will obviously reduce the weight, although if you are going to really crank the amp a Gold might be marginal for power handling.