$100 Dilemma: Revive Childhood 9V, or Go Powered Up?

I pulled my 90s collection out of storage for my train-obsessed 4-year-old, and the nostalgia hit hard. I have the 4559 Cargo Railway and 3225 Classic Train, both in great shape. The catch, however, is I'm missing the 9V Train Connection Wire. Replacements appear to be $50–$100 (!!!) I'm torn on the best path forward for a kiddo who is just starting their Lego journey: (1) Invest in 9V: Buy the wire and keep the 9V dream alive, OR… (2) Modernize / Powered Up: Put that $100 toward a modern...