I have a friend who builds everything. He can machine parts etc. Several years ago he looked into building a mud motor. At the time he had a 20hp Honda beaver tail mud motor. I had a 23hp Briggs Go Devil and a 16hp Briggs Go Devil. He took measurements from all of them, messed around with endless research and parts lists. He finally landed on a medium kit from SPS Swamp Runner. He said there was no way to come close to $$ building it on his own. Actually he is now a SPS dealer and puts kits together for people who don't want to mess with it. He has sold a lot, lot, lot of them with no trouble. lol
I live on the river and he occasionally has customers test drive SPS's out here. We have also tested many of their prototype motors. I have owned, driven or been in almost every long tail out there over the last 20+ years. From the small boat Golf ball mentioned and where it will be used, I would strongly recommend the small SPS kit with a 6.5hp Predator motor. One person can pull the shaft off in seconds and carry motor up & down steep banks in two pieces. Often when trapping, with the SPS's in various sizes I own, I pull the shaft off and bring it in shop or tent overnight to keep grease warm so pull starting is real easy in am. I have also loosened the shaft and pulled it out far enough to disengage, then start and warm up motor on real cold mornings. Turn off motor, re engage shaft and re start motor, your on your way. Props are cheap and can be replaced in under 5 min. I used to run an 8" prop up stream checking traps, then swap to the 8 1/2" or 9" back down stream with my medium kit, just to get a little more out of her. Many years I can go all season on the same prop, other times I'll break one the first week. I have never used more than 3 in one season.
Few main things are:
1. Always wear kill switch tether and life jacket with mud motors. Make sure dogs and onlookers, or anything else you don't want chopped up are clear from prop area b4 starting. I don't like to take my eye off running prop when people or dogs are around.
1. Balance boat front to back and side to side. You do not want to run with bow up and transom down like a traditional outboard. I like to have a seat I can move a bit forward or back, the rear fixed seat in most boats is to far back and to low for long tail motors imo. I often send my dog forward or back while cruising to keep the boat in the "Sweet Spot". I also will move a Beaver, couple Coons, gas can etc, here or there to balance to Sweet Spot" while cruising. Makes a huge difference. The smaller the boat the more important this is. You will feel it when you get it right.
2. Keep prop in middle of wake. When cruising, prop should be in center of wake. Adjust, by bending skigg (fin below prop) after balancing boat, to keep prop in highest part of wake wash while cruising.
3. Run the boat for several days or weeks b4 taking it out for work. There is a bit of a learning curve and you don't want to be messing with that while running traps or set lines. Do not turn real sharp at high speed until you get used to it. There is increased leverage on handle while turning.
Good Luck and be safe out there!