I have a tf magna that i got for 100 bucks. it had a bit of a rattle in the engine. i have been able to find a cheaper 3.5 than 3.0 ltr can they be swapped

Yes they can! Just make sure you get all the accessory brackets, fuel rail transfer line etc... See here: http://www.aussiemagna.com/forums/showthread.php?t=64826&p=1143879#post1143879

Strip the engine down and isolate the rattle. Might be something minor. Another engine will have its own problems. BUT I believe such a transplant is possible, not having done one myself I don't know what other bits need replacing. Others can answer that.

I would never replace an engine without finding out first if it is terminal, and a replacement engine I would strip down and check its condition before doing such a swap. Why spend such an amount of time and money and risk having a still wounded engine in your car at the end of it? Risky unless you know its a low km engine that has recently come out of a car. If the 3.5 has been sitting around for a while, it will need a new cam belt, valve stem seals and a clean out of the hydraulic lifters as a minimum.

i cleaned the engine out with diesel and replaced the oil and the rattle stopped (suspect low oil was the cause) but was thinking that the 3.5 would put out better performace. the 3.0 only has 190,000 k on it and the 3.5 is alot more. i just thought i would ask the question

Hydraulic lifters can rattle if the car hasn't been used for a while. At 190,000 Km it has lots of life left in it. The 3.0 revs out better but the 3.5 has more low down torque. I prefer revs! If you got the car for $100 I imagine the body work or paint is poor, maybe spend your time and money on that?

yeah it need a few parts and a bit of work but i have it just bout all ready for the raod now all for under 2k so it is working out well worth the money coz the car is work over 5 k up here