The game doesn't actually crash as much as you think: read to find out how you can play more of the game
it's very lucky that runouw built cheats into this game:while on the file select screen, press the letters "c, h, e, a, t" and it should bring up the cheats menu, click both options and then start a new file. you can continue as normal through the tutorial, but before you enter the castle, pause the game, click star map. click and hold the down arrow on the map to scroll until you see storyline shines: click on the first two shine sprites (they should turn yellow), then unpause. you should then be able to pause, click exit, and exit level. which will bring you into the castle
This game runs fairly smoothly once you get past the door and it's not that hard either, I am a professional speedrunner of this game, and I've experimented with different emulators before. Some of them work immediately, others are more similar to this emulator (called ruffle as I'm sure you've seen). I'm sure you've all seen, the credits are synced to the music; This is actually the case with every room in the game as well. However, ruffle... doesn't support music positioning yet so some rooms can definitely break.
Now onto how you can fix this problem, the "lag" you experience is because ruffle can't compute marios position. It's not technically the game lagging it's just ruffle not being able to place objects correctly. For some reason, holding pause (the P key) while entering a room fixes this problem, however in some cases (like entering the castle) this still doesn't fully fix the problem, instead you will have to wait until you can see the star shape appear, then hold p for... something like 10...15 seconds?(this will probably depend on your computer too) it should make mario go through a secret passageway (you may never have known about this passageway before because it's hidden pretty well) which will takes you to the endless staircase. Now sadly, Bowsers castle is completely broken and I'm fairly sure it can't be fixed so you can't actually complete the game, (for now, there are actually secret dev pipes which runouw never removed one of which is a pipe which leads to the final escape[last 3 rooms of the game] that we MAY be able to access via very complicated glitches and inputs but that's beside the point, and has yet to be discovered) so instead, go to the opening to the left, it should lead to the door to exit the staircase, now again this will be a room where ruffle struggles, so hold p as you are entering and it will (for some reason) take you to galaxy of stars.
Heads up: this is a pretty hard level, you have to collect 5 silver stars then make your way to collect the completed shine, all while having 0 fludds and dealing with intense lag(this level is pretty laggy even on the original game). If that wasn't hard enough already, because ruffle is broken you basically have to do it without dying once or you will have to do the whole thing over again from a new file. Thankfully the game autosaves so depending on how long you play, you are guaranteed to keep at least enough progress where you can now progress without having to start a new file. Now basically the best thing you can do is to hold p through every (and I mean every) transition as some of them kill you, and a few of them kill you whether you hold p or not. You may want to look up the WR (world record) for any% in this game as it will show you where to find secret passages which will allow you to navigate more quickly, and more safely. Finally: remember if you ever go through a laggy transition like entering the castle, either wait until you can see part of the room and then hold the p button until the next frame, or just hold p until the framerate smooths, then exit the level. Other than that I think the only things that will help you is learning speedrun tricks/glitches and skips.
Alot of you probably want to play/speedrun it without it being broken, we will be able to help you download flash easier if you join the discord, the way the easiest way to access the discord is by going to
speedrun.com/sm63 and looking around for the discord page, (if I remember correctly it might be in the resources page)
I hope this helps some of you who want to play this game again. I look forward to seeing you in the community if you decide to join :)
also watch out for the download super mario comments, pretty sure they are all bots :/ most of it promoting their own crap