Thanks a lot Flack RNinMunich and Tica! I tried to read me up on Li-pos, but was even more confused... Your answers is just what I needed! 👍👍👍

It's a great old-timer ( it was my boy dream back in time, BTW s... I'm getting old 😱)
Stay with 540's and go for LiPo's, for the difference in weight vs capacity you will get a very long time on the water.
But you need to get a proper high quality balancing charger to take care of your LiPo's.
I have changed to LiPo's in almost all my models.

Hi Yngvegr, basically I agree with Flack👍
But if you want to go step by step and test it with the 540s first (will give you a Datum to compare with brushless at a later stage) then-
Brushed have no problem with LiPos, or any other battery chemistry for that matter😉
For the voltage anything from 6 to 12V is fine for the 540.
I have two 540s in my 135cm destroyer and she ran just fine on a 6VSLA, with a relay to switch a second one in series for 'All Ahead Emergency'!
Higher capacity brings not only more sailing time but also more weight!
Suggest you weigh the existing batteries and check the waterline and trim in the 'domestic test tank' before you buy the biggest batts you can find😉
If you do go LiPo don't forget to buy an appropriate charger and a Capacity Tester as well!!!
Whatever you do, have fun doing it, cheers Doug 😎

I think that if you are considering changing to LiPo's then it is worth the complete transition and also move to brushless motors and appropriate speed controllers. However if you don't wish to go to the expense involved in such a move an appropriate battery size in Lipo would be 7.4v, the higher the capacity in mah will only increase the an mount of time it will last. I think the performance will prove to be exciting.