The front hub and brake look wrong. The Tiger 90 only ever had a 7" SLS front brake and yours has an 8" TLS item. If fitted properly it's a very desirable improvement. See if your local library can find you a copy of Roy Bacon's Triumph Twin Restoration as it should help you spot significant changes from standard.
Uprating the engine from 350cc to 500 cc is straightforward but it has to come completely apart as the conrods are different to accommodate different gudgeon pin diameters. Again, no problem if done by someone who knows what he's doing.
It's a very tidy motorcycle. I'm completely out of touch with the prices these machines fetch so my only suggestion would be so skim the small ads at the end of a few magazines to see what's currently on offer.