I am looking for a set of plans for a guy weighing about 300 pounds for simple local flights or even a two seater. Ive looked at a lot of plans but it seems designers think the human race all come in at 250 below.
Modifications like this are of course possible but you also have to look at the way it's built, the materials and gauges around the cockpit, the interior reinforcement, etc. One of the issues to address with a bigger cockpit payload is not only the flight loads but also those of the person getting in and out. One misstep or catching oneself if misbalanced and the load of grabbing on could be more than enough to bend things. So yes, you can convert a two seat small plane to a single seater (I've always wanted to do this with a Globe Swift) but a look at secondary loading scenarios would definitely be a worthwhile exercise. Also, despite having two seats, the planes like the KR-2s tended to perform poorly with a larger payload so it may be a good idea to look at the larger planes where the performance differential would be minimized.