90 gt 145k miles what to do

What's your budget look like? Are you doing the work yourself?

First, I would check the PCV valve and make sure that screen isn't clogged. If that goes wrong it will make these engine's spew oil all over the place. If that's good, this can be a sign of piston ring blow-by. I would do a compression test, and a cylinder leakdown test, and see how it stacks up. The fact is these engine's when properly cared for are usually good for at least 200K miles. However, you are going to have to pretty much have to tear it down to the shortblock anyways. If you are getting excessive blow-by then a re-seal will have a short term fix. If the system is good, it might just be the fact you have almost 22 year old seals that have just died of old age. You could pull the motor and tear it down to the shortblock, replace all seals and gaskets. It's going to be hard to justify putting back some of that stock stuff. If you are going to do all that, and the ring seal is still real good, might as well do an explorer top end. If the bottom end is poor or marginal shape, just swap over an explorer engine. Upgrade the valve springs, put in the stock or better cam, and you'll be happy you did. All options above this are going to be out of your budget.

put some nitrous on it, that will fix it.

If it were me I would do all new gaskets, intake manifold, valve cover gaskets, timing cover, front main, oil pan and rear main. Then I would do a full tune up, cap, rotor, plugs, plug wires, fuel filter and then see how you sit after that. You can buy a full gasket kit for less than 100 bucks from 50resto. Then the tune up stuff will be like 120 depending on what you buy for everything.