I'd never 'built up' a wheel before until 2023. Decided to give it a go, had bought NOS tubular alloy rims & eBay'd Campy hubs identical to what was on my MBGR at the time I bought it in 1972. Bought new spokes & nipples, then an eBay'd Park truing rig (not really necessary given that others suggest it can be done as well while mounted on a bike frame) 'cause I'm kinda a 'tool guy', why I also bought a Park tensioning device.

Really, you have a wheel that's already assembled. Pulling a spoke would be my first step so you can measure its length and gage the diameter(s). Then simply count how many you want to order up & go looking for a source.

Knowing what length you need is vastly better than trying to compute it for a newbie! I'd made a mistake on my first order, it cost me some significant cash.

Were I riding your saddle I wouldn't hesitate now to replace all the spokes on that wheel....