There is an aftermarket option (at least one that I am aware of).
A company called CMC makes after market trim packages. They have one for 35 horse and under models (amongst their line up) but it's not cheap - in the $600 range - sold at places like Bass Pro.
The unit is also not "neat" in terms of size ect - your best bet is always "buy what you want up front". This unit will add about 25 pounds to the back of your boat. There may be other options.
Personally, I would put up with the inconvenience of having to tilt the motor manually or replace your current motor - the new 20 horse Merc's "have" power trim. The 20 Merc (now) is built by Tohatsu and they also market their own model if you want to price compare.
As to the question of "is it enough motor", the simple answer is NO. I understand you are operating on a "restricted lake", so 20 horses is your "max". Your boat itself has a hull weight of about 600 pounds - by the time it's rigged and you (and whoever) is onboard with your "stuff" you might be looking at 1200 to 1500 pounds that the motor has to push - a 20 is just "never going to get up and go" no matter what you do to it.
Pushing all that weight will require a prop with "less pitch" then what would be considered "standard" - a 20 would normally be on the back of my 200 pound 14 foot aluminum, not a 16 3/4' 600 pounder. So while less pitch will allow the motor to "operate" in it's designed range (as far as rpms go), you won't get any more speed out of it.
Your particular boat was "designed" to be powered with (minimally) a 35 horse motor (to get it to perform as designed) or more practically, 40 horses (it's max rating).
If you are going to operate your rig under the restrictions you mention, there just isn't much you can do to make it work "like you want it to" - it's just not capable...