What headsets can be used to replace the FSA No.57/No.61BP headset?
I received an email from FSA customer support:

Thanks for contacting us.
Headset No.57/61BP is a cheeper headset with retainer bearing, we don’t have this Headset in stock and also this type of bearing as spare parts.

We suggest to install the below upgrade version, so in the future you can replace only the bearing:
130-0026            HS SX Pro (I/I) 15mm alloy 1-1/8 1.5
Hope this can help.
Ciao
Stefano

Googling this, I found a headset called FSA Gravity HS No. 55 SX Pro, which has the following specs:

Upper headtube ID 44mm
Lower headtube ID 54.92mm (Follow-up email indicates that this is in fact 55)
1 1/8&quot; to 1.5&quot; tapered steerer
50-61mm headtube outer diameter