My old Target trailer has vacuum over hydraulic breaks. I would like to convert this to either electric over hydraulic, or surge brakes. Could I possibly change the tongue to accept a surge actuator, or would the electric over hydraulic thing be better? It has huge brakes on all 3 axles, boat is a 40' excalibur. The trailer has been repainted since this pic, I know, it looks rough!

If you are going through all that work, I'd convert it to Electric/Hydraulic, and forget about surge. WAY too much work for surge. As long as your coupler is sound, you should be able to get everything you need from www.championtrailerparts.com