I would like to know what are the major obstacles if I try to build a new table saw. My major problem today is that my DeWalt 745 has an aluminium top. I was wondering if I stand any chance to move the motor to a cast iron top that from another table saw (I just saw one for sale on ebay from a Rigid 4512)
I don't know about the guts of the Ridgid saw, but I can imagine that a switcheroo isn't going to be simple. (Probably not impossible, just not simple.)
However, I have to question your motivations.
size of top? Simple -- just build a big table that you can drop the dewalt into
fence capacity? Not quite as simple, but not impossible to put a new Biesemeyer-type fence and rail on a table. (Spendy, though, if you buy new...)
mass? I know the dewalt is light and thus squirrely, but there are a number of ways of weighing it down. If you tend you use it in a table, then bolt it on and weight the table legs. If it's on the ground, you can bolt it to a long hunk of ply (which will also help with tipping).
Last but not least, the dewalt has value if you were to sell it. If you really want something more substantial, you might just sell it and buy what you want.