...Possibly changing the rudder profile to more of todays Cap 232 look. Have no idea how it will change performance (if good or bad...) or if by changing the rudder will I no longer be able to fly it in IMAC Sportsman / Intermediate due to the Scale rules?
As long as you stick to the 10% rule you're fine.
I had a flying buddy who had one of these - it definitely was heavy, but it flew precision aerobatics well. I believe he had a SuperTigre 2300 in it, don't think a DA50 would be a good choice for your ship - but you can certainly try! I seem to recall it had relatively small control surfaces compared to the newer designs.
As long as the Sportsman/Intermediate forms don't have too many vertical lines and you watch your downline speed, you should do fine in the classes you mentioned.
DB