Just fix the compression issue on the 305. Nothing else would be worth your while. If I really wanted an engine replacement, it would be a full-on project. Personally I'd go for one of the 454 or 500 water-cooled twins and try and make it fit. The 305s are quite rare now, I think. Much easier and cheaper to repair it than do anything else.
You've never tried to fix a compression issue on a 305 in recent years. New jugs are unavailable, 99% of used ones need rebore, Oversized pistons are next to impossible to find in pairs, only one comes up every couple years. I've checked overseas, underseas, in the sky, cross-compatibility and nothing, zilch zip nada. There was a new 1.0 on ebay for a while (might still be) but he wanted $140 for it and one doesn't do anyone any good. You'd pay that much again for 1.0 rings, if you found em, and still be a cylinder short
KaosKontrol post=866850 userid=62324 said: I'm not skilled enough to rebuild the engine myself
yes you are, if you can find the parts. I've never rebuilt anything bigger than a toaster and with the help of this site I rebuilt my top end (farming out the valve/machine work). Unfortunately it ended not being enough because the cylinder was a tad too scored, but it did run when I put it all back together. Wouldn't go above 45 tho and blew smoke. For the same reasons you are stuck, mine has been stuck 2 years waiting to find parts. So you can definitely do it.