What type of motor swap are you talking about? 3.8-3.8 or 3.8-4.6 or 3.8 to an oldschool 5.0? 135000 miles are actually not a lot and you should not have any issues on the car. My 3.8 has 187000 miles right now on it and all ive had to do is normal maintenance as well as some gaskets, but while i did gaskets i did timing chain, tensioner, cam, springs, pushrods and rocker arms. That was all a "just cause it was apart" thing. The only way to "significantly" improve the performance on the 3.8 is to supercharge it. My cam and windstar swap did not bump it up that much, although a guy who did similar things as i did produced 238rwhp and like 248rwtq. Another guy who did similar things as i did produced like 400rwhp and 438rwtq but he dropped a supercharger into it and changed the crank to make it a 4.3L instead.