Welcome to the forum. There are ready made replacement hardware packs available, here's one:
Sick Bike Parts
This kit is less expensive than buying the individual parts from a hardware store.
If you decide to go the Allen head cap screw route go for grade 5 or better still, grade 8 fasteners. Do Not use stainles steel.
I'm sorry I don't have a list in front of me with all the sizes but most are 8mm and varying lengths. If your engine is not installed yet it's not a big deal to just take it to the store with you. You'll need to estimate the required length by having everything attached, i.e. engine mounts, intake manifold and exhaust pipe/muffler. I use Allen head cap screws for almost everything on the bike and engine. Good luck. Send us pictures of your completed build.