Hi Steve - There are 2 different types of exhaust on the sandcasts - the 'early' no stamp ones, and the later no stamped ones (i.e. no hm300). If you have a look in the restoration advice on this website it shows the difference with photos (just discovered it myself a couple of days ago).
I think also, the seat which yamiya now sells are the very early sandcast seatpan style - less holes. again - this is in the restoration advice. I bought one of the earlier seats with the K1 seatpan which looks more like the K0 seatpan, with the extra holes for the spring near the seat catch.
yes, I had to cut the carb top rubber gaskets also - I just laid the original over the top and traced around the inside of the cable hole. use a very sharp pen knife, or even better, a pointed scalpel blade and dont slip (dont worry about cutting a finger - they heal! if you have to order from yamiya you will always buy more than you need ;-) ) good luck!
steven.