I have a supply of sheet lead that I cut and use. Would look the same as the lead shot in your pic but you won't need much. I ballasted mine with all the gear in the hull before the deck was added, got it all working then fitted the deck etc. I left it slightly high on the waterline to allow for the top bits.
Modern rc gear is now easily available in small form and even the batteries can be sourced in small sizes. I used AA pencells in the model and it sailed for well over an hour, The ESC had a BEC, my own design so shrink wrapped to keep the weight down.

Yes I see what you mean about needing removable rigging. You opened the deck exactly where I figured would be best do the job. The new mini rc gear similar to the Footy hardware will fit nicely. A small brushed motor would be my choice too.

Did you use lead shot for ballast? Such in the attached photo?

Hi, I have the same boat that I bought a couple of years ago as a uncompleted static model for my model railway. I was going to motorize it and put R//c gear into it but had no ideas as to how! after seeing this post I can now go ahead and do the modifications and will be able to use it on the small pond I am setting the railway around>😜

Yes you can. it's small so not much room to get access and the rigging needs to be removeable.
I have one that I converted almost 20 years ago and it sailed OK. Should be easier to fit out now as the rc gear, ESC and batteries are available in much smaller formats.
Should fit well in your small pond, I don't really have anywhere safe to sail mine, hence the disrepaiur.
I am attaching some photos of its present condition. Needs some restoration but hopefully shows what is possible.
I used a small maxxon motor and a large brass prop over scale but it worked. The acess is via the removable cabin and the battery fits in a tray towards the front. The rx and esc fitted above the rudder servo.
Have fun and please share your build.