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In addition to what Ukaflyer said (check and meter all the wiring), look specifically at the new fingers on the reverse unit. Almost all replacement fingers are slightly long, requiring the contact tip to be bent back to make contact with the brass inserts on the drum. In the third picture (end on view) the finger on the right looks like it could be too long to hit the contact on the drum.

Was the field winding open circuit that it was replaced? How do you know the armature is good? We always try swapping with a known good armature to verify.

When you connected Super Flex leads to your field leads did you first bare the ends of field lead wires of their protective coating?  As you check all circuits for continuity (as already recommended) see if you have it across your field (and if so what is the resistance in ohms).  A 220 grit Dremel paddle-wheel polisher does a great job on a number of things, one of which is to prep surfaces for soldering (such as removing field wire coating).  Be careful of the direction it spins and wipe any surface before and after with 91% isopropyl alcohol.   One other thing of late, the rivets on reverse unit fingers are not to be entirely trusted.  The slightest gap can exist ruining an otherwise perfectly good reverse unit overhaul (and drive you nuts until you find it).  Typically all you had to do was be certain of a good through-hole soldering (not just on the top surface).  Now I "super solder" every finger before use.  On the underside of the board, lightly polish where the rivet and finger meet along the back edge of the board with the Dremel 220 paddle-wheel, wipe with 91%, apply flux, and solder that spot to ensure positive and lasting connection between the rivet and the finger.  I tried ensuring that connection with a rivet driver, but found nothing works as well as "super soldering".  No problems with any done that way prior to install.  It is similar to how they're unfortunately made with "pad" finger overreach.  Adjusting (bending back) the throw of top and bottom "pad" fingers to ensure proper contact as pads come into position is, as mentioned earlier, key along with of course proper contact pressure by all fingers on the drum all the way around the drum (drums are not always perfectly round).  While the engine is running a reverse unit should trigger with about 10VACr (10 VAC measured at the railhead with a load).  Finger pressure (along with other factors) contributes to how much power is required to advance the unit.  Optimally dialed in units will function well and dependably with as little as 8VACr.  I initially worst case bench test mine with the motor, lights, etc. also active in the circuit with a lowly (but beloved) 1-1/2B.  If it a reverse unit functions well at the bench with the red arrow knob of the 1-1/2B at "10" then it predictably works well at 10VACr or less while running on the layout with a 30B.  Not to say your issue(s) is solely due to the reverse unit, but as something that plays a critical role in an engine's performance.  Check, as was mentioned, every circuit (including how well each segment wire of the armature is soldered to its respective commutator post).  Once resolved, as was mentioned, a 300 in good tune will run glassy smooth and spry.  I did one a week or two ago.  One of over 100 engines repaired or completely restored (mostly the latter as with that 300) in the past 12 months.  Scads more prior to that.  

Good luck.  You'll get it figured out.  

Be well and have fun running your trains,

Dave

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