my 91 ln130 surf is a bit saggy in the back end esp when ive got a trailer attached, has anyone got any part numbers for some heavy duty coil springs or has anyone used some off another vehicle that would do the trick?????

ive got a 1991 ln130 surf with the 2.4 turbo diesel, its getting rather tired and am looking at something with a big more power, the body etc is in great nick still so thought an engine repower might be the go, has anyone repowed the power plant in one of these, what engine could i chuck in and ...