I think you can add the recoil starters on the trx420 ?, it's optional, but deff can be put on them.You must be in Canada. 420's in the US don't have recoil starters.
Yes, oil will come out of the hole. Might not be a lot, but crankcase pressure will sling some out.
And again, running it with nothing in the hole is going to get all kinds of dust and crap in your motor, and that kind of contamination will RUIN your motor, so unless you want to rebuild the entire thing, do NOT run it with a big open hole in the back of it!