What engine will fit my 1990 525i e34

Depends how adventurous you are feeling and what you mean by 'fit'.

Cheap E39 540i touring on ebay with rot and gearbox problems. A great engine!

www.ebay.co.uk/itm/BMW-540i-touring-E39-/161976895779?hash=item25b6934923:g:V9gAAOSwDuJWvvQn

No.
First off, the v8 is out. Completely different braking system, cooling system, engine bay layout... everything. Unless you have a complete donor car and even then it's annoying and time consuming. People have done it but then people have fitted sr2o engines in them too.
If you want to keep it easy, sort of...
Is it an m20 or an m50?
An m20 can be upgraded to an m30. The gearbox and prop will need changed and either the diff and Shafts or you can change the diff flange but it will only last if you don't try any burnout. 735 is the same as 535 but again the m30 does not fit on the other gearboxes. Or if m20 you can upgrade to m50.
If m50 a bigger m50 is the way to go. A later m52 engine gets you a 2.8 option but there is a few bits and pieces to do to make that work, or a later 3.0 6 cyl from an e39 if you are feeling adventurous with the electrics.

The M52 is the weapon of choice in the E30 world, have a look on the E30 Zone website for detailed technical info on that swap. Not sure how much that helps with your E34 but will give you a good indication of the general swap.

The M20 would be a backwards step. Great engine in an E30, but not what you're looking for. Unless you want to go the turbo route in which case again see E30 Zone for details on how to do it, there's quite a few turbo'd E30s out there!

The M52 is the more stable choice I'd say, but a turbo'd E34 could be a hoot.

This:



Is rather answered by this:




It depends on whether you currently have an M20 or M50 E34

M20:



M50:

M3 engine in Australia is an absolute fortune. Dear enough here, but there...

OP the M50 is the six cylinder engine in 2.0 and 2.5 capacity, built from 1990-1996, with a steel block and oil cap at the rear of the head as shown in the above picture.
M52 is the replacement 2.0, 2.3 and 2.8 engine built 1995-1998, with mainly an alloy block except for 1995 examples which were pig iron, and the oil cap at the front end of the rocker cover.
Essentially the same bare engine but different ancillaries and sensors and wiring.

But really the advice from now on really will need to be based on what engine you are starting from, so we would need to know what you have...

750i/850i engine. Preferably on three sets of XJR1300 bike carbs.

The 528i will be an M52. More than likely being a '99 car will make it a double vanos (variable timing) M52TU engine. Those you want to avoid, the wiring is difficult to get them into an E34.

So you have the 170bhp M20B25 engine you reckon. From there it is not too awkward to go to the 192bhp M50B25 or indeed with slightly more work the 193bhp M52B28. What you'd really want to do there is pick up a cheap knackered M50 2.5 engine and a cheap 528i, as the E39 has a different sump and you'd need the M50 one, plus you can fit the M50 loom and ECU to keep it simple, rather than trying to adapt the E39 loom which is also different enough to the E34 one to be annoying. M50/52 engines will bolt to your transmission (although the M20 box is a bit weak for the M5x torque), or you can change the box and prop over if you fancy, not too hard.
or, ignore all that, and go straight to the 212-215bhp M30B35. This needs to be from an E34 535i or E32 735i. The E28/E24 etc cars with this engine use different sumps, pipework, looms etc. One from the E32/4 will set right in. You will need to use the bigger radiator, and all the ancillaries and engine bay attachments, but otherwise straight forward. You will have no choice but to use the M30 gearbox as your m20 one will not fit, and you will need an M30 E34 (530i or 535i) prop - 7 series one is longer. You would also be well advised to change the differential to the 535i one which also means changing the subframe mounting bar and the half shafts. The M20 ones will not sustain the torque of an M30 for long. However the M20 diff is the same fitment as an E30 one so you can raise some cash from it to cover the expense.
Exhausts are all different but are all twin pipe systems so you can figure out what you want to do there as you see fit.