Do not force the shifter lever to go into gear without trike moving. Unless you want to get up close and personal with the insides of the 1800 engine. Better yet, just get the stock shifter lever put back on again.
Are you married to those floorboards or would you just as soon go to stock?
I ask because I started out with stock pegs etc on my trike, then went with Add On Low-boys to straighten m y left knee, and they had a H&T shifter which I dispised, so I kept the FBs and eliminated the H&T shifter making my own shifter and adding a left side heel rest for assisting with shifting. Very pleased. Don't know what you have on 1800, mine is a 1500, just offer as an idea.