I have a couple of unreleased fixes that you may or may not be interested in including. The first is fixing the menu statistics for some engine-swapped cars, for example a V8-swapped GT-R being reported as upwards of 900hp when it only has ~580hp, or a Works rotary MX5 with a similarly large gap. I never got around to testing that upgraded power figures are more accurate too, or checking all of the Works cars, but it's a start. The numbers aren't exact either, but a 5-10hp ballpark is a lot better than 350-400hp.
The other fix is a potential way to provide reward cars for sale without actually removing them as a career reward. I'm in the early stages of testing it however, and there will need to be some pricing tweaks as most prizes are priced at $100,000, which is generally cheaper than building a car to beat the series. I'm tempted to just cut it down to unlocking the more 'generic' cars, ie. the Exige, Z06, MP4-12C, Z4 GT3, M3 GT2 and GT-R GT1, leaving the specific cars like the RTR-X or Twins Viper as unique prizes. This means that series-dominating cars like the drag Scion still require you to build a car that can beat them, but cars that players may want multiple of (Works and non-Works Exige, or different-liveried GT-Rs) can still be bought. The Falken 997 I'd probably leave as prize-only too, since you can already buy a GT3-RSR.
I've also recently made this
tweak to the Speedhunters DLC which I'd class as a fix, but mainly because I hate DLC that forces itself on the player.
Out of interest, can I ask what it was that was breaking saves in older versions of the patch? I've found the game to be very resilient with saves, including coping with level changes and deletion of garage cars, so I'd quite like to see what can kill it!