You're best off replacing the service equipment at this point
Unfortunately, the type and vintage of your existing equipment, along with the facts that you have a bonded neutral generator and that UL Classified breakers are no good in Canada, means that you're best off replacing your service equipment now, instead of trying to cobble something together now and plan for a big upgrade later.
In particular, because your panel accepts type TR and type TQL breakers, but not type THQL breakers, you run up against a recent design change to the THQL breaker line, documented in this DIYChatroom post (quoted below for posterity):
GE changed the way they set the breaker's seating height. In my panel, you push the breaker down until the top of the molded slot rests on the stab. TQL breakers, THQL Issue UOxxx (probably MJxxx, NExxx and others too), and also Siemens QP and Eaton BR, can work this way, because they all have the same slot profile.
Recent panels have a ridge of insulating material next to the stab; the bottom of the breaker's case rests on the ridge. The stab is not as tall. It doesn't penetrate the breaker as deep. All the above breakers (except TQL which somehow doesn't quite fit) can be mounted. Also Issue RT-xxx of THQL, which doesn't fit the old bus because it can't accommodate the tall stab. Issue RT-xxx is all you will find in retail stock.
As a result of that, and the fact that UL Classification is no good in Canada (which means you can't legally use Eaton CL breakers), finding any sort of feeder breaker to put in your panel is going to be difficult at best. (You'd have to scour grey-market sources for used or New Old Stock THQL breakers that predate issue RTxxx.) Furthermore, your plan with putting a transfer switch upstream of the service equipment doesn't work either due to the Canadian requirement for a separate service wiring compartment in service equipment, something your proposed transfer switch doesn't meet.
While some Siemens generator panels made for the Canadian market do support being wired as service entrance hardware, you're better off getting a Canadian-market combination generator panel that has two sections of breakers, one for non-generator-backed loads and the other for standby loads, in it. This gives you a clean installation and room for future upgrades to a 200A service and/or a larger genset, while eliminating the breaker supply troubles you're having at the moment. Furthermore, these panels have a switching neutral function, which works nicely with the bonded neutral on your generator to avoid headaches involving wayward neutral current, tripped GFCIs, and so on.