Hi Paul and welcome to the Aero S1 fold, the original and the best (?) Yes the original battery was a gel battery but a lead acid will suffice, gel probably fitted for safety reasons in case you rolled the car. I can recommend Tayna batteries of Abergele, plenty of choice for you on their website, Ref your silencer Librands used to do a sports system so maybe worth an enquiry. I look forward to your photos and progress with the recommissioning .

1969 4/4
1995 plus 8
1996 Plus 4 4seater
2002 Aero S1
2013 M3W
2014 Plus 8